Tuesday, April 5, 2016

Variables in the Publix Marathon

In class this semester, we learned about two different types of variables. Variables may be continuous or counted. Counted variables are, perhaps obviously, things we can count. They are represented by ordinal numbers. Let’s look at these two types of variables in a real life example: the Publix marathon. The Publix marathon took place in Atlanta a few weeks ago. There were 1,378 participants. There were 909 male participants. The number of people is a counted variable. What continuous variables are there in the Publix marathon? Time is a continuous variable.

You might think time isn’t a continuous variable because you can count it. You can tell me what time it is: “It’s 12:30”. Looking at the difference between second and third place overall winners (which can be found here), minutes are specific enough to separate them. There are 13 minutes separating these two runners. Sometimes though, time needs to be more specific than just hours and minutes. The 9th and 10th place finishers overall, Jacob and Jeffery Law are separated by 1 second. These two, who appear to be brothers, ran the whole race together with the same pace of 6:46 per mile but in the end one of them had to finish before the other. Since time is continuous, Jacob, the older brother, gets to brag about beating his brother in a marathon.

Still, we all learned that a second is “one Mississippi” in kindergarten. We can find things that are measured in less than a second by turning away from an everyday example and back towards science. In my lab, to determine enzyme kinetics we use a rapid flow quench machine. 

This machine allows for a minimal reaction time of 2.5 milliseconds! An example of the data generated from this machine can be seen below where one student in our lab collected five data points in 0.5 seconds.

Whether you are trying to win a marathon or determine the kinetics of your favorite enzyme, the fact that time is a continuous variable is a good for you. 


  1. Thinking about time as a variable is really interesting for me, since I'm about to embark on a live cell imaging project. We know that the biology I'm studying is relatively slow over all, but some of the dynamics may happen very rapidly--we really have no idea since there's nothing published on what we're trying to measure. To this end, I have to decide on a time frame (how long am I going to take measurements for?) and a frequency (how often will I take images?). I may capture very different information if I take an image every 2 minutes over the course of an hour versus every hour over the course of a day, with roughly the same number of total images! Interfacing between a discrete counted variable such as the degree of order that I want to measure and a continuous variable like time is quite challenging. If I could, I would mimic a continuous variable by imaging rapidly for a long duration to observe the biology I'm investigating nearly continuously, but alas we are always limited by the lifetime of our fluors to discrete moments in time.

  2. Emily, I actually just recently dealt with a very similar problem on a live cell imaging experiment! We were trying to measure specific events during mitosis in two different cell populations. Imaging the cell cycle is tricky since nothing happens for ~23 hours and then BAM! all of the intricate details of mitosis happen very rapidly as the cell divides in about 30 minutes. I did an initial video analysis with images taken every 7 minutes for 24h and saw some interesting phenomenon. However, when I repeated the experiment with images every 2 minutes for 24h, I saw much more detail as to the order of certain mitotic defects that I couldn't see with imaged every 7 minutes. When trying to quantify discrete countable events that are only 3-5 minutes long out of a 24h video, it feels so strange to treat a time frame of 2 minutes as a continuous variable, but alas time is still continuous. Ideally it would be better to decrease my image acquisition time even more so that time more closely resembles a continuous variable, but again, capturing those rare events is so difficult to do without killing the cells from phototoxicity. Glad to know I'm not the only one with imaging timing struggles!