Tips from a Physics Tutor Doing the Experiment - Measuring Your Bias

Now that we've brainstormed our sources of bias, how do we measure them? Let's make the results of our experiments as accurate as possible! Welcome back to our discussion of experimental error with a physics tutor. Whenever you are conducting experiments in your high school physics or college physics courses, you need to consider the sources of error that might throw off your result, and you should at least be aware of the methods you might use to mitigate the effects of these errors. An appreciation of fundamental experimental techniques and terminology will also serve you well as you hone your study skills or prepare to tackle the physics GRE or SAT2. Let’s pick up where we left off! In our last post, we began discussing the sneaky problem of bias in your experiment. Unlike noise, which causes your measured value to jump randomly from run to run, bias, which skews all of your results in the same direction, is more difficult to see in the data. We used the example of trying to measure the height of the average male basketball player. If you measure every player in the country but forgot to ask them to take off their shoes before stepping up to the measuring tape, then all of your measurements, as well as your final result, will be high by an inch or more. So then, how do we make sure that we don’t forget the shoes or catch our mistake after the fact? If some gremlin in your experiment is throwing off all of your data by the same amount, then how do you detect this shift? The first step to fighting bias is identifying it. Even though this may seem funny coming from a physics tutor, for this first brainstorming step, let your imagination take over! Try to look at everything in your experiment with fresh eyes. In our last post, we returned to our original experiment – measuring g by timing how long a ball takes to fall from a fixed height – and brainstormed a number of ways in which our measurement of the time could be biased by how we are performing the experiment. Now that we have drawn up our list of possible culprits, how can we go about measuring how much of an effect each source of bias has on our result? Let’s look at two methods you can use. Method 1: Switch it up! When you are trying to test whether some part of the experiment impacts your result, often the easiest thing to do is to try different variations of the experiment that change up the part in question. For instance, there’s the question of whether you or your partner has faster reflexes and if that could bias your results. There’s a simple way to answer this question: switch places! Are you saying â€Å"go† and your partner is reacting? Try having your partner say go! Who drops the ball and who clicks the stopwatch? Try it both ways! Notice also that many of these choices are independent; that is, there are four different combinations of you and your partner holding the ball or the stopwatch, and saying â€Å"go† or reacting, and you can explore each one. If you try enough permutations and are careful to keep track of what parameter you are switching, then you will begin to get a feel for which effects matter and which don’t. It might very well turn out that it doesn’t matter if you drop the ball and say â€Å"go† or your partner drops the ball and say â€Å"go,† but it matters a great deal (maybe 0.2 s or so) if the person holding the stopwatch or the person holding the ball says â€Å"go.† This seems like an awful lot of work, though, doesn’t it? Well, the truth is that it is. Take for example the decades-long quest to measure the electron dipole moment, which is essentially asking if the electron’s mass is centered at precisely the same point as the electron’s charge. These experiments, including the current record holderand a collaboration that is gunning for the leadare absolutely state-of-the-art precision measurements. They might spend days or weeks running the experiment at full speed in order to collect enough data to drive down their statistical uncertainty (effect of noise), but they spend years trying to nail down their systematic uncertainty (effect of bias). In the record-holding experiment, for example, they identify 9 different quantities, things like their electric and magnetic fields, that could conceivably influence their result. They come up with 512 different permutations of these parameters and test every single one. This is essentially a sophisticated (and thankfully automated) version of the procedure I propose for our humble measurement of gravity. Method 2: Worst-case scenario What if there is some part of the experiment that you just can’t switch off? Some approximations – that none of your strings have mass, none of your wires have resistance, and there is not a molecule of air in your physics classroom – can never (within reason for an introductory physics class) be completely true. Then how do you deal with this inevitable imperfection, the effects of which are probably (but might not be!) negligibly small? What do you do if you want to measure something small with precision? You make it bigger! Take the question of air resistance. What if the surface of your ball is so rough that the air resistance noticeably slows the ball as it falls? This would result in a longer drop time and an incorrectly low estimate for g. How can we make this effect much worse? We could try placing a fan below the ball blowing up – that air resistance is surely much greater than what we would see with still air. That gives us one bound for the effect of air resistance: the time we would measure if we were dropping the ball in a vacuum should be no greater than the time we measure when fighting against a headwind. What about the other bound? Let’s try moving that fan so that it blows down on the ball: the time we would measure in a vacuum should be no shorter than the time we measure when being pushed along by a tailwind. If the times we measure under these different circumstances are the same, down to the uncertainty imposed by drop-to-drop noise, then there we go: air resistance probably does not matter. If they differ by, say, 0.2 s (after accounting for noise), then we can say with confidence that air resistance in still air doesn’t push our result one way or the other by more than this 0.2 s. You can find a way to make almost anything about your experiment worse. Worried about the mass of that string? Try using a thicker one. Worried about resistance in that wire? Try using one five times as long. We spend a good deal of time trying to make our experimental setup better, so go ahead and have fun making it worse. Putting it all together By now, you have a good idea of how much thought can go into making even a simple measurement as accurately and precisely as possible. Broadly speaking, we can perform the experiment many times and average the results to drive down the impact of noise on our result as much as possible. We can be creative with how we perform the experiment in order to tease out the various sources of bias that might be lurking in our experimental procedure. This process gives us a result and uncertainty for one quantity: the time it takes for the ball to fall to the floor, in this case. In the next post, we’ll look at how you take the results and uncertainties for every quantity you measure during the experiment and combine them to get your final result. Stay tuned!

A UAMS M3 Shares Her Med School Experiences

This interview is the latest in an Accepted blog series featuring  interviews with medical school applicants and students, offering readers a behind-the-scenes look at  top medical schools and the med school application process. And now, introducing Carly†¦ Accepted: Wed like to get to know you! Where are you from? Where and what did you study as an undergrad? Carly: Growing up, I have lived all over the South. For the past several years, however, my family has lived in Arkansas and I’ve grown to love it here! I went to Hendrix College in Conway, Arkansas for undergrad and majored in Biochemistry with a minor in Accounting. Kind of random, I know, but I wanted to learn more about business and finance in case I decided to open my own medical practice one day. I actually really enjoyed my accounting classes much more than my â€Å"science-driven† brain initially thought I would.   Accepted: Can you share three fun facts about yourself? Carly: 1.  I love to travel! My mom is a food and travel blogger so we have had the opportunity to travel all over the country. My favorite place to go is Alaska! The scenery there is so beautiful and breathtaking. We are going to Alaska again this summer and I could not be more excited!! 2.  I am the oldest of five kids in my family with the youngest two being adopted from Russia, so I have learned to speak a little Russian. 3.  I am a dog lover through and through! My family’s five year old black lab, Cocoa Bean, always keeps us on our toes.   Accepted: Where are you currently in med school? What year?   Carly:   I am currently a third year medical student at The University of Arkansas for Medical Sciences (UAMS) in Little Rock.   Accepted: Why did you choose this program?  What is your favorite thing about that program? Is there anything you’d change? Carly:  I went to undergrad less than an hour away from UAMS and so our premed program was very involved with the school. I was always interested in how much the med students who came to talk with us really seemed to love being there. As the only medical school in the state, it is a decently sized school but still retains a hometown feel. I love how personable the professors and attendings are and how much they really care about the students doing well. Over the past three years, my favorite thing about my school has been my classmates. After going through the same trials and experiences that are all a part of med school together, we have grown to become a very close-knit group. The only slight problem we’ve had has been with our curriculum. We were the first class to go to a new curriculum format at our school so there have been a few kinks to work out, but I think a lot of these have been figured out in the last couple of years and I really think the new curriculum is greatly helping everyone learn better and more efficiently. Accepted: Whats your favorite clerkship so far? Carly: My favorite clerkship has been Pediatrics! I always thought I would like to specialize in pediatrics but wanted to be sure I actually really liked it before deciding. Luckily, I did! I love the cheerful and friendly atmosphere of a children’s hospital and really enjoy spending time with kids. Accepted: Looking back at the application process, what would you say was your greatest challenge? How would you advise other applicants who may be experiencing similar challenges? Carly: The greatest challenge for me definitely was writing my personal statement. I have always found it hard to talk about myself and so found it pretty difficult to write about what I had done as a premed. Eventually, I started to shift my thinking more towards trying to express to whomever would read my personal statement why I was passionate about medicine and wanted to become a physician. I found this much easier to talk about and was able to share my heart on the paper. Accepted: Whats your position on the recent petition to cancel the Step 2 CS?   Carly: I definitely understand the argument here. Not only is the test ridiculously expensive and inconvenient with having to travel to one of only five testing centers across the country, but it also is somewhat redundant to what many medical schools are doing already. My school has a great clinical skills program and we have had many clinical skills exams during the past three years. All of them have been modeled exactly how CS is done with specific, individual feedback from both the standardized patients on our communication skills and from attendings and faculty on our clinical reasoning and approach. I know that many medical schools also have their own internal clinical skills exams to test their students’ communication and clinical competence and feel that these are extremely helpful in regards to how much personal feedback can be given on this individual level. Accepted: Can you tell us about your blog? Who is your target audience? What have you gained from the blog experience? Carly:  I started blogging during my junior year of undergrad. I was becoming overwhelmed with my upcoming research project, MCAT prep, application season, and so on, and was getting really stressed out. I started looking online for any tips and encouragement and then started getting the idea of making my own blog to share what I had (and was) learning on this journey to becoming a doctor. At first, I didn’t feel qualified enough to share any advice or tips with others since I was just going through it myself, but I also wanted to have an outlet to share with others who might be going through the same thing. I have loved blogging! I have been able to write about many topics from premed classes and the med school application process to more recently about my third year clerkships. While I haven’t been able to post and share as much as I would like, I have loved being able to reflect on and share my experiences with my audience. In the past year, I have really been getting into Instagram. I love how quick it is to share what I’m doing on a particular day and can share much more frequently than I am able to on my blog. You can follow Carlys med school adventure by checking out her blog or Instagram (@doctorsoftomorrow). Thank you Carly for sharing your story with us! For one-on-one guidance on your med school applications, please see our catalog of med school admissions services. Do you want to be featured in Accepteds blog? If you want to share your med school journey with the world (or at least with our readers), email us at bloggers@accepted.com. Related Resources: †¢Ã‚  Med School Kicks Off: Ten Tips to Get You Through the Season †¢Ã‚  Med School Admissions Resources †¢Ã‚  The Best 4 Things to do Before Med School