The 4 components of an “Experimental Study”

1. The 4 componentsof an “Experimental Study” Start 

2. A little terminology Yesterday we learned a new meaning for an old word, “population”. Today we are going to encounter a few more, beginning with the START ingredient of the previous slide. The ingredient has two “somethings”, and we already know what to call the one on the right ….. …. yes, the population! Observe that the population can be made out of anything(people, dogs, poppy seeds, wolves,…) Start 

3. A member of the population in an experimental study is called an “experimental unit.” These two terms, population and experimental unit cover pretty much the “something” on the right. It is essential at the start of an experimental study to define precisely what constitutes the population of interest, lack of clarity will lead at best to confusion, at worst to grievious errors. I will give you two examples, both from real life. (one is from my real life, it comes from WW II, the other happened in this country a long time ago.)

4. Variables The “something” on the left represents some characteristic of the population about which you want to make some statement. For example, (for each example I want the population) the heightof people in this class, the favorite color of seniors at ND, the number of hours that a first year student at Podunk University spends online, the yearly snowfall in South Bend, the quality of maple syrupharvested from trees in Maine, how many wolves per square mile can be found in Wyoming.

5. In all of these examples the characteristic you are interested in may vary from experimental unit to experimental unit (the height changes from person to person, a square mile in Yellowstone will have more wolves than a square mile in Laramie, the yearly snowfall in South Bend this year is a lot more than many other years in the past and probably in the future….) This explains why we call such a characteristic a variable.(it varies, duh!) Definition.A variable is a characteristic of interest in an experimental study.

6. Samples So far we have seen the following ingredients of an experimental study: • A population of interest • One (or more) characteristic(s) of interest that vary from experimental unit to experimental unit. If the population is not too big the easy way out is to check the characteristic of interest for every experimental unit. (this is called a census.) Not quite!

7. What if checking the characteristic destroys the experimental unit ? (how do you check the functionality of a match?) or is frightfully expensive ? (how do you check the functionality of a rocket?) or may take forever ? (how do you checkthe life-length of a NeverReady battery?)

8. In most instances a census is impractical, in others (political surveys) one tries to predict the outcome of a census without actually performing it (the census is performed … on election day ! ) Instead of performing censuses (actually censi) experimental studies rely on samples which consist simply of subsets of the population. Definition. A SAMPLE is a subset of the population. We should now consider the first component of an experimental study, collecting data