How to answer “design-an-experiment” questions on biology exams.

[I’m still re-reading Genome and my next post was going to be a discussion of the importance of metaphors to thought, with a particular focus on how that should impact teaching. Unfortunately, it’s crunch time for both me and my students–coming up on the end of the quarter for them, and I am working on the outline for my master’s thesis as well as a small freelance project. My mind is drowning in questions of grades and assessments as well as the data for my thesis, and I haven’t been able to do enough reading to pound out the post I wanted to write. Hopefully this will be at least a decent substitute.]

A staple of well-written science exams, particularly at the higher levels (though it should be far more common earlier in science education–a topic for another day) is the design-an-experiment question. I am a HUGE fan of this question style, because it allows you to assess understanding of a variety of concepts at the same time, and it requires higher-level thinking. In any subject, it is easy to design a course that packs lots of facts into the heads of college students eager for good grades. The perpetual problem of teaching is how to go beyond that–how to teach and assess skills, particularly critical-thinking.

My students are supposed to graduate with science degrees within the next year. I teach parasitology. Parasites are awesome, but knowledge of the nitty-gritty of their inner workings will hardly be an essential skill for many of my students. And frankly, while a science degree should imply that one has a certain amount of basic knowledge stashed away, that isn’t worth much if it doesn’t imply a basic understanding of the scientific process. Designing experiments and interpreting the results–that’s the scientific process. It’s absolutely essential to a biology education. It is also, when you first start, incredibly intimidating.


Alright, so enough of my rambling–how do you answer a question like this? Here’s my guide:


It’s easy to look at a dense paragraph at the top of an otherwise blank page awaiting your answer… and freak out a little bit. If you’re panicking, please remind yourself that these questions are usually goldmines of partial credit. You do not need to be perfect or know everything about science to answer one of these questions. In a lot of cases, graders will even accept or at least give partial-credit for answers that include technically unfeasible suggestions so long as they are logically sound. The point here is to see whether you can take your scientific knowledge and put it to use in a meaningful way, not to see whether you can walk into a lab tomorrow and know every little detail necessary to actually do it.

2. Read the damn question. Carefully. Determine what precisely what question you are being asked to address and what information you are being given.

This seems obvious, but in a long question that you’re reading in an anxious hurry, it can be easy to miss crucial details. You need to know exactly what your experiments are going to have to show, and exactly what you have to work with. Note as well what information you DON’T have. For example: are you being asked to answer a question about a hypothetical or unkn0wn-to-you organism? Then you can’t assume you have the genome sequenced, or the ability to do complex genetics, unless the question tells you that you do.

3. Consider methods/techniques available to you.

Step one here would be to rule OUT any techniques that are in some way prohibited by the content of the question. Basically, consider your limitations.

After that, you should be going through a catalog of every experimental technique directly discussed in your course, as well as all the relevant basic techniques you have learned in previous courses. Which ones give you the type of output required for the question?

For instance, if the question requires you to determine whether a given gene/protein is required for a given process, you may consider making a knockout mutant. If the question requires you to determine which [currently unknown] genes are involved in a given pathway, you may consider random mutagenesis followed by a screen for mutants that have defects in that pathway. You also will need to consider what inputs various techniques require. If you propose making a knockout, you need to know exactly what gene you are looking at and have the ability to do basic genetics in the organism. If you want to do random mutagenesis to find genes relevant to a given process, you need a way to screen your mutants.

4. Put together techniques to answer the question as well as you can.

Explain this in a logical sequence. ‘I would do A, which would tell me B/give me product B to be used in next step. Then I would take B and…” Do the best you can. If you can’t get all the way to the answer, but you can get to a necessary intermediate step and then explain in more general terms what you would need to get from that step to the final answer, then write that. Remember to logically connect your steps to each other. Don’t require your grader to make assumptions about what you mean–be explicit.

5. Explicitly explain how to interpret your results, making sure that they answer the original question.

This seems obvious, but you’d be surprised how often students answer exam questions with almost everything correct, but fail to explicitly connect what they did back to the question that was asked. Often, these students come to me later and complain about having lost points for this, because obviously I know what they meant. They don’t get the points back though, because I can’t make assumptions about they meant. This isn’t some huge arduous task. Just look back at the question, and explain in a sentence or two how your experiment answers it (“If X is true, then I would expect to see result Y” statements work pretty nicely for this most of the time). Easy peasy.

Things to avoid:

  • Writing up five different experiments if you were asked for one. If one of those is correct and the others are wrong, you’ll most likely get few if any points. I understand the impulse to throw everything including the kitchen sink at a question like this, particularly if you’re unsure, but it won’t actually help you with points most of the time.
  • Making lots of [unreasonable] assumptions. In a well-written question, all the information you need will either be in the question or should be something you were taught in class.  If your experiment requires you to assume that you have a lot of information that was not referenced in the question then, most likely, you are on the wrong track.  That said, not all questions are perfectly written. If you feel you can only answer if you make a few assumptions, at least be explicit about what you are assuming.
  • Being disorganized/confusing. We try very hard to follow your logic, but we aren’t mind readers. Something as simple as writing in list form or giving a flow chart or picture will express your point better than a long, jumbled essay.


And that’s it! See guys, easy, right?

Just kidding. It is a lot. Designing an experiment, even a hypothetical one for an exam, generally requires more creative thought and more synthesis of concepts/information than most other types of questions. But it isn’t magic, and it isn’t beyond your reach, even if you’ve never worked in a lab or otherwise feel you are lacking in experimental expertise. Remember the basics. Have controls. Vary one variable at a time (as much as possible, anyhow). Keep things as simple as possible, don’t use long convoluted methods if you don’t have to.

Science is complicated, yes, but the actual process of doing science is about breaking down that complexity into simple, well-defined questions, and answering a lot of those until you add up to bigger answers. And yes, sometimes in the real world that requires coming up with Brand New Ideas/Techniques that no one has ever tried before… most exam questions do not. These questions require piecing together a bunch of stuff you already know, and about sometimes about being willing to put yourself out there enough to propose something you aren’t 100% sure of. Relax. It’s just an exam question, no one’s actually asking you to cure cancer or anything.


Now, I’m exhausted, probably because I already taught this three times today, along with the parasitology material I was actually supposed to be covering, and I’d like to wrap up now. But there is one more problem to be addressed: How do you actually learn to do this? What is the best way to study to prepare for this type of question?

Many of my students ask me what techniques they should be familiar with, hoping, I suppose, that I can give them a list to memorize and then they’ll be safe. But I really can’t do that, for several reasons. On a practical level, that list would just be so damn long at this point in a biology education, because students are supposed to retain knowledge from previous classes. From a high-minded perspective, I shouldn’t need to give a list because the whole point of this type of question is for the student to demonstrate that they can own their knowledge and put it to use, not that they can memorize more things. But finally, even having a list wouldn’t necessarily be enough. You have to have of a feel for the material and for the logic of experimental design to really excel at this type of question.

I can tell you the best way to gain that nebulous ‘feel for’ designing experiments, but you may not like it, because it isn’t a nice bullet-pointed list of clear tasks. The best way to get comfortable with designing experiments is to think a lot about experiments. Read a lot of papers, pick them apart in seminars or journal club. Work with a complex paper for hours until you understand completely the logic behind the experiments and conclusions, repeat endlessly. Work in a lab, with real questions to answer and real experiments to participate in designing. Work through example questions with a friend, brainstorming together all the possible options and discuss why each would or wouldn’t work. Ask people who are already good at this about how they think about it. Basically, put in the time, challenge yourself, and practice. No shortcuts.

Realistically, I can’t give that advice to a student who is 7 or 8 weeks into a 10-week quarter and desperately trying to get good enough at this to pass, or to maintain their A, or whatever. In the short-term, what I can say is: read any papers required for the course, and understand them. Know any techniques discussed in class. Get your hands on some old exams or example questions and work through practice questions with a TA or tutor or classmate.  And follow my tips above, of course. These things alone can definitely provide meaningful improvement on a short timeframe if you’re dedicated.

But in the grand scheme of things, if you want to do this well, if you really want to be able to prove you can think like a scientist, there is no checklist of tasks I can give you that, when complete, will guarantee your preparedness.  You have to build your own deep knowledge over time, and you have to figure out the way of doing that that works for you.


In summary–

  • Don’t make assumptions, and don’t require your grader to make assumptions about what you mean. Be explicit, fill in the blanks, spell it out for us.
  • Don’t expect perfection. Embrace partial credit. Make educated guesses. Have a little faith in yourself.
  • In the short term, practice makes perfect.  Seek help where ever you need to get it, but study hard and practice a lot.
  • In the long term, work towards building deep knowledge of and general comfort with experiments by exposing yourself to them. This is what science is all about–pushing the boundaries of what we know, not memorizing what we’ve already found out. It’s terrifying, but it’s also thrilling. And sometimes even beautiful. Remember to step back from exams every once in awhile and remember what science is for.

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