Critical Thinking Examples In Science

* Note: it is page 43 in the 6th edition

Dany S. Adams, Department of Biology, Smith College, Northampton, MA 01063


We all expect our students to come away from our classes knowing some of the facts; but more importantly we want our students to come away knowing how to think critically. Less clear is how to teach the process, perhaps because few of us learned it explicitly , perhaps because for those of us who make it to the level of teacher, critical thinking was in some sense intuitive and automatic. This is not the case for the majority of students.
The good news is that because the scientific method is a formalization of critical thinking, it can be used as a simple model that removes critical thinking from the realm of the intuitive and puts it at the center of a straightforward, easily implemented, teaching strategy. I describe here the techniques I use to help students practice their thinking skills. These techniques are simply an expansion of the Evidence and Antibodies Sidelight in Gilbert's Developmental Biology (2000, Sinauer Associates); that is, I harp on correlation, necessity, and sufficiency, and the kinds of experiments required to gather each type of evidence. In my own class, an upper division Developmental Biology lecture class, I use these techniques, which include both verbal and written reinforcement, to encourage students to evaluate claims about cause and effect, that is, to distinguish between correlation and causation; however, I believe that with very slight modifications, these tricks can be applied in a much greater array of situations.



This is a poster about how I tweak my Developmental Biology lectures so that in addition to learning facts, concepts, and certain key experiments, the students learn the principles of the scientific method, and go away able to apply the thought process in other contexts. Because the scientific method is just a formalization of critical thinking, that means that the students become critical thinkers. And that is what I most want to teach.

The basic idea:
Explicitly discussing the logic and the thought processes that inform experimental methods works better than hoping students will "get it" if they hear enough experiments described.

How I do it:
I devote three lecture periods to explicit discussions of observations, loss of function and gain of function experiments, and controls. This introduces the first principles and the vocabulary of experimental biology. Thereafter, every piece of information can be, and frequently is, discussed with reference to those principles. Every one of those discussions, the final project, and all the tests, reinforce the same ideas.

What the students get out of it:
1. They understand where information comes from.
2. They know where to start when you ask them to think about something.
3. They understand experiments, both classical and modern.
4. They can read the primary literature and comprehend much more, more quickly.
5. They can judge the validity of conclusions.
6. Every student seems to get it, even those who are not stellar.
7. As their confidence grows, they become more active participants in class.
8. They are AWARE that they are thinking well, and most find that very exciting.


YES. I am impressed over an over again by the improvement in my students' ability to UNDERSTAND the primary literature, to ASSESS the validity of claims, and to THINK critically about how to answer questions.


The majority of students respond very positively; others are neutral. I have not encountered anyone who found it a negative experience. On their mid semester self evaluations, students wrote the following statements in response to the question "Where would you say you have shown the most change for the better?"

"I believe that I am gaining a real understanding of how to go about asking questions...The experimental design techniques and problem solving approaches have really strengthened my critical thinking skills"

"It's becoming easier to read complicated journal articles with understanding"

"I ... like the experiment section of the test because I can apply my knowledge."

"The research proposal was really difficult for me...but that's good, it means it's a challenge."

"[My] critical thinking has expanded... experimental thinking has made science in general more clear for me. I feel less overwhelmed by all the research and knowledge by understanding how to break it down into manageable questions."

I have also received the following spontaneous comments:

"I am studying pathogenic E. coli for one of my other classes and am reading this book on the microbes. I came across this paragraph, part of which I have to share with you!! It talks about how... 'the intimin of E. coli was shown to be NECESSARY BUT NOT SUFFICIENT to induce lesions.' I just thought it was so cool that I am reading this highly scientific book and can make sense of concepts that would have been so foreign to me not all that long ago!!"

One student actually expressed regret that the fourth exam was the last...

Does it take much work to incorporate this?
NO. Especially given the pay off.

Advance preparation: Some work the first two years, then none.
It took about 15 minutes to add the blurb to the syllabus. I devote 1 � 2 lectures in the first or second week of the class to a careful examination of the experiments described on page 25. During those lectures we talk about necessity and sufficiency, and why you need both kinds of evidence, and I introduce the short hand SHOW IT BLOCK IT MOVE IT. Another lecture, further into the semester, is devoted to controls. Now that those lectures are written, my preparation is minimal.

During lecture: Less work than before.
Like everybody, I was already talking about experiments in lecture; this is merely a modification in how I talk about experiments. Having the SHOW IT BLOCK IT MOVE IT vocabulary in fact saves time. What used to take 10 minutes to describe now takes about 5 minutes: the students understand the whole picture much more quickly since they already understand what experiments can, and can not, tell you.

Exams: Less prep work, more grading work
Writing exams takes half the time. 50% of every exam is prepared simply by finding an appropriate observation and describing it. Having done this for three years, I now have a collection of good observations so there is even less work. Grading exams does take longer. I strongly encourage students to write very succinctly, but this is, admittedly, the one downside of this approach.

Anything else?
This turns out to be the best strategy I've ever found when asked a question that I can not answer. My old approach was to be completely honest about my ignorance, say "what a good question", and "where do you think you could find the answer to that ?" Now, I am completely honest about my ignorance, then I turn the question into a class-wide discussion about how to design experiments to answer the question. It turns a potentially useless moment into an opportunity for the students to practice thinking.

What I do DURING LECTURE and how it is different from what I used to do?

I. In the syllabus is a blurb warning the students that they will be asked to think about the experimental basis of knowledge. I read this out loud during the first class. Difference: it takes an extra two minutes.

II. Sometime during the first two weeks of class, I devote two classes to a detailed discussion of the experiments described on page 25.
I begin with the life cycle of Dictyostelium discoideum. Difference: none.
Next we talk about the observations and the hypotheses they engendered. Difference: time is devoted to an explicit discussion about what observations and hypotheses are, and how they differ from experiments and facts.
Then we cover how antibodies work and how they are used. Difference: they learn about this technique earlier in the semester.
Then we discuss correlations. We give the nickname "SHOW IT" to the category of experiment that shows correlations. Difference: Again, time is devoted to an explicit discussion of correlative evidence. I do not have to hope that they know or will pick up the difference between correlation and causation.
Next we discuss loss of function evidence. We give the nickname "BLOCK IT" to that category of experiment. We talk about how a block it experiment shows necessity.
To introduce the last kind of evidence, we talk about the limitations of block it experiments, and we discuss how something can be necessary but not sufficient.
Next is gain of function evidence. We use "MOVE IT" as our nickname for that category of experiment. We talk about how a move it experiment shows sufficiency. We also talk about how something can be sufficient but not necessary.
Finally, I reiterate, and the class discusses, how all three types of evidence are needed to show cause and effect.

III. After that, any experiment that comes up in class is immediately put into a category that the students already understand. Difference:
It saves a huge amount of class time.
It provides instant context for any experiment that comes up.
it gives the students examples of, and practice at, critical thinking.
The students don't just hear experiments, they UNDERSTAND THEM and how the results fit into the big picture.

IV. I also provide the students with an empty "tool box". Every time a technique is mentioned in class, we pull out the toolbox and write notes about the technique in the appropriate box. Difference: by the end of the semester, the students have been introduced to, and thought about how to use,, an impressive number of techniques, and they UNDERSTAND the power and the limitations of those techniques. On a very practical level, they end up with a list of techniques and controls they can consult in the future.

V. Toward the middle of the semester, I devote an entire lecture to controls, including why you do them and how you do them. From then on, when we talk about a technique, we also talk about the appropriate controls, and we add them to the tool box. Difference: students actually UNDERSTAND controls.

VI. Finally, EVERY TEST has a gradually growing question, always worth 50%, that asks the students to make a hypothesis about an unfamiliar observation then design experiments to test the hypothesis:
TEST 1 � Asks for the hypothesis and three experiments
TEST 2 � Asks for the hypothesis and three experiments
+ consistent & inconsistent results for all three experiments
TEST 3 - Asks for the hypothesis and three experiments
+ consistent & inconsistent results for all three experiments
+ controls
TEST 4 - Asks for the hypothesis and three experiments
+ consistent & inconsistent results for all three experiments
+ controls
+ alternative hypotheses

Because the question grows, and because the early tests count for a smaller percentage of the final grade, the students quickly recover from their anxiety about a "new kind of exam", and actually begin to enjoy (?!) solving the puzzle. Difference: THE STUDENTS PRACTICE THEIR CRITICAL THINKING SKILLS in a way that is, for the students, fun and memorable (because of the constant reinforcement) and for me, simple and reasonable easy to evaluate.
The Course
Developmental Biology, once known as embryology, is the study of how organisms and the cells that comprise them change and grow through the life cycle. For most of the organisms we will study, that cycle comprises the development of the organism from gametes to adults that produce more gametes, (exceptions to this cycle make a marvelous study). In addition to being a fascinating and aesthetically pleasing subject, modern Developmental Biology represents a synthesis of many of the subjects you have already studied, including Cell Biology, Genetics, Evolution, and even a little tiny bit of Physics. Thus you will be reviewing, reinforcing, and remixing many of the concepts you have already learned in other classes. I believe that you will find development to be an exciting context in which to think about cell behaviors, biochemical reactions, and forces.

One of the great joys of being a scientist is that your view of the world is constantly changing. Sometimes those changes are quite profound - remember learning that objects were made of molecules ? - others are more subtle. One of my goals for this course is to offer you a new way of seeing living things; that is, I hope that you will begin to appreciate the incredible but true stories behind the ability of mighty oaks to grow from tiny acorns.

Another important component of this course will be the emphasis on putting information in the context of the scientific method. In other words, we will structure our study with reference to the process of making observations, followed by formulating hypotheses, then testing of those hypotheses, analyzing the results of the experiments, and forming both conclusions and new questions based on those results. In fact, all of the tests will have one question in common: there will be an observation, and you will be asked to make a hypothesis, describe experiments to test that hypothesis, make predictions about the results of the experiment, and discuss the results. We will also use this framework as a guide to interpreting experiments and understanding how those experiments contribute to our current understanding about how organisms develop.

Developmental biologists are still seeking answers to questions first asked by embryologists at the turn of the century. To understand the extraordinary, not to mention currently trendy revolution that is going on in this field, you must first see what the original embryologists saw when they watched organisms develop. In other words, you must watch organisms develop. Thus we will spend time studying the observations that others have made, and making many of our own. The rest of our time will be devoted to understanding how things happen - that is, we will study the mechanisms underlying what we observe, at least, the ones we think we understand.
PAGE 25 (43 in sixtth edition)

Reproduced with permission of the author

Hypothesized Cause

MethodologyPositive Controls
(To compare with negative results; to show that methodology works)
Negative Controls
(To compare with positive results; to show that methodology does not confound results)
ProteinImmunocytochemistryWestern Blot w/ pure protein Stain known positive cellsPre-immune serum
2nd Ab only
No treatment control - To show what the organisms would have looked like if they'd been left untouched


Hypothesized Cause

MethodologyPositive Controls
(To compare with negative results; to show that methodology works)
Negative Controls
(To compare with positive results; to show that methodology does not confound results)
TissueRemove tissueStain for marker
Remove then return
No treatment control - To show what the organisms would have looked like if they'd been left untouched


Hypothesized Cause

MethodologyPositive Controls
(To compare with negative results; to show that methodology works)
Negative Controls
(To compare with positive results; to show that methodology does not confound results)
DNATransfect gene (w/inducible promoter & reporter)Look for reporter; northern &/or westernTransfect with neutral DNA
No treatment control - To show what the organisms would have looked like if they'd been left untouched
EXPERIMENT QUESTION FROM TEST #3, approximately 9 weeks into the semester.
The Observation:

This cartoon (fig. 19.1 of Gilbert Developmental Biology, 2000; reproduced with permission of the author) shows the early cleavages of the nematode worm Parascaris aequorum. What is illustrated is that a special cytoplasm, termed the germ plasm, is segregated to particular daughter cells. Cells that do not inherit the germ plasm, undergo a process called chromosome diminution, which means that the chromosomes start to fragment. (Aside: this is an interesting exception to the rule that all the cells in an adult animal have the same genes present). The germ cells are all descended from the cell that does inherit germ plasm and that retains its full complement of DNA.
[1] Make a hypothesis about a process (the cause) that might be responsible for some aspect of the phenomenon (the effect) described above.

[2a] Describe an experiment to determine if the process and the phenomenon are correlated, either in time or in space (correlation; "show it").
[2b] Describe a result that is consistent with your hypothesis.
[2c] Describe a result that is inconsistent with your hypothesis.

[3a] Describe an experiment to determine if the causative process you have hypothesized is necessary for that aspect of the phenomenon to occur (loss of function; "block it").
[3b] Describe a result that is consistent with your hypothesis.
[3c] Describe a result that is inconsistent with your hypothesis.

[4a] Describe an experiment to determine if the causative process you have hypothesized is sufficient to cause that aspect of the phenomenon to happen (gain of function; "move it").
[4b] Describe a result that is consistent with your hypothesis. [4c] Describe a result that is inconsistent with your hypothesis.

[5] Describe a control for ONE of the above experiments, and state what you are controlling for.

(reprinted with permission from the author)

[1] Hypothesis
There is a protein [that I will call] PCFP, [that is] found in germ plasm [and] that prevents chromosomal fragmentation.

[2] Correlation ("show it")
Produce an antibody to PCFP and expose cells of the germ plasm and the cells that do not inherit germ plasm to the antibody. [Use a] secondary antibody conjugated to a [fluorophore to image the primary].
Consistent result: germ plasm is stained with antibody and the cells that do not [inherit germ plasm] are not stained.
Inconsistent result: Germ plasm does not stain with antibody, or, both germ plasm and cells without germ plasm stain.

[5] Control
Positive control to determine if antibody is working: Identify cells that are known to contain PCFP; expose these cells to the antibody.

[3] Loss of function ("block it")
Identify the gene that encodes PCFP and perform site directed mutagenesis on that gene.
Consistent result: germ cells have fragmented DNA.
Inconsistent result: germs cells still have no chromosomal fragmentation.

[4] Gain of function ("move it")
Introduce a plasmid into cells that don't inherit germ plasm. This plasmid will contain the gene encoding PCFP adjacent to [a] galactose inducible promoter.
Consistent result: in the presence of galactose, these cells will not have chromosomal fragmentation; in absence of galactose, they will have fragmentation.
Inconsistent result: in presence of galactose, the cells will have chromosomal fragmentation.
It has happened more than once that an observation given on an exam has turned up in the primary literature soon thereafter. In all cases, students had designed experiments that matched the published work. When I bring those papers into class and show the students that their proposals match science that is actually being done and published, they get a tremendous kick out of it. (They also find it very satisfying to find out what the result actually is).
All reproductions from Gilbert, S.F. (2000) Developmental Biology 6th Edition are reproduced with permission from Sinauer Associates, Inc. Sunderland, MA.

These examples of critical thinking show how we can apply the ASK framework to evaluate scientific claims.

ASK about the person’s Authority on the topic, whether their Starting point is biblical or humanistic, and how they Know what they claim to know.

In the previous article we looked at a framework for evaluating truth claims and thinking critically about any new idea that comes to you. To summarize, when you hear a truth claim, you can ASK about the person’s Authority on the topic, whether their Starting point is biblical or humanistic, and how they Know what they claim to know.

Using this mnemonic device, you can learn how to stop and ASK the right when evaluating any truth claim, regardless of its source. As you learn, you can be passing these ideas on to those you are discipling, teaching them to look to Scripture as the ultimate authority in every area of life.

Scientific Claims

You likely get your scientific information from sources that digest it for you. There are many popular outlets for scientific information about the latest fossil finds or medical research; most major news outlets provide a section for such ideas, while some websites are devoted to such news. In general, the authors of these articles examine an article or other item published in a scientific journal like Science or The Journal of the American Medical Association and then presents the information in the article along with some quotes from the authors, and sometimes a contrary point of view, to explain what the find means. However, even those who claim to be fair and balanced are not necessarily friends of the Bible. Let’s look at a few examples of critical thinking and put the ASK framework into practice.

Of Asteroids and Dinosaurs

Headline: Asteroid may have killed dinosaurs quicker than scientists thought (Reuters, February 2012)

Here are a few samplings from the article that you can use to evaluate the claim:

Earth’s climate may have been at a tipping point when a massive asteroid smashed into what is now Mexico’s Yucatan Peninsula and triggered cooling temperatures that wiped out the dinosaurs, researchers said.

The time between the asteroid’s arrival, marked by a 110-mile-(180-km-)wide crater near Chicxulub, Mexico, and the dinosaurs’ demise was believed to be as long as 300,000 years.

The study, based on high-precision radiometric dating techniques, said the events occurred within 33,000 years of each other.

The theory that the dinosaurs’ extinction about 66 million years ago was linked to an asteroid impact was first proposed in 1980. The biggest piece of evidence was the so-called Chicxulub (pronounced “cheek’-she-loob”) crater off the Yucatan coast in Mexico.

Renne and colleagues reanalyzed both the dinosaur extinction date and the crater formation event and found they occurred within a much tighter window in time than previously known. The study looked at tektites from Haiti, tied to the asteroid impact site, and volcanic ash from the Hell Creek Formation in Montana, a source of many dinosaur fossils.

About 1 million years before the impact, Earth experienced six abrupt shifts in temperature of more than 2 degrees in continental mean annual temperatures, according to research cited by Renne and his co-authors.

Begin by thinking critically about what claim the author is trying to get you to accept: The author wants me to believe . . . that dinosaurs died out 66 million years ago after a period of climate change and then a meteorite impact.

Authority: Is the information coming from someone with authority on the topic? To answer this question you would have to dig a bit to find out more about the authors, but the paper has been published in Science by authors with degrees in geology. So, the author is using a source that does have a degree of authority on the topic.

Starting Points: Does the author have a biblical worldview or a humanistic worldview? Clues in the article point to a humanistic worldview as the starting point. First, dates that are far outside the biblical age of the earth are given as fact. Second, there is no mention of God or the Bible as a source of truth or an attempt to compare what was stated to the Bible.

Know: How does the author know the things he claims to be true? The conclusions come from an analysis of radiometric and climate data. The article claims that six shifts in global temperature of more than two degrees happened across 1 million years more than 66 million years ago. This is an example of an interpretation of data, not observational evidence. No one measured the temperatures with a thermometer—they were interpreted from the actual observations, they are not data themselves. It seems quite unreasonable that such accurate temperatures could be determined, and the analysis is based on uniformitarian assumptions that are unbiblical and unconfirmable. Sorting data from interpretations is an important part of evaluating truth claims. Another interesting aspect is that the article claims that the new window of time was much tighter than “previously known.” So what was known before is no longer known, demonstrating the spurious nature of what is actually known about what happened “millions of years ago.”

Final Analysis: This claim must be rejected even though the authors are authorities since the idea contradicts the Bible and is an interpretation based on faulty thinking.

Of Mice-like Critters and Men

Headline: Meet Your Mama: First Ancestor of All Placental Mammals Revealed

A tiny, furry-tailed creature is the most complete picture yet as to what the ancestor of mice, elephants, lions, tigers, bears, whales, bats and humans once looked like, researchers say.

These new findings also suggest this forerunner of most mammals appeared shortly after the catastrophe that ended the age of dinosaurs, scientists added.

From all this data from living and extinct mammals, the scientists extrapolated the appearance of the most recent common ancestor of all placental mammals.

“We have all these placentals alive today, from elephants to shrews, from things that fly to things that swim,” Spaulding said. “What could the common ancestor of these things that are so different possibly look like?” The scientists then worked with an artist to illustrate this ancestor.

In brief, the author wants us to believe that all placental mammals came from an ancestor that looked like this picture. Let’s ASK some critical thinking questions.

Authority: The article mentions a team of top-notch scientists with expertise in their fields.

Starting Points: The author and researchers clearly have a humanistic worldview, rejecting the biblical explanation of the origin of living things, including humans.

Know: The picture and recreations are not based on actual observations, but a blending of molecular analysis and comparisons of fossils that are put into a beautiful picture through the mind of an artist. Notice phrases in the article that demonstrate a lack of concrete knowledge, such as these: “speculate on the appearance,” “may have looked like,” “much remains controversial,” “fossil evidence suggests,” “genetics research suggests,” “possibly look like,” “the researchers suggest,” and “supports the notion.”

Final Analysis: This idea must be rejected because the ideas contradict Scripture and the conclusions are based on speculation of evolutionary relationships with a sprinkle of artistic flourish.

The next time you check the headlines in your news feed, take some time to practice this framework. Read through the article and assess the claims while you think critically and ASK questions. Then, take the time to teach it to someone else.

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