Chapter 8 Threats to Your Experiment

Chapter 8 Threats to Your Experiment Planning to avoid criticism. One of the main goals of this book is to encourage you to think from the point of view of an experimenter, because other points of view, such as that of a reader of scientific articles or a consumer of scientific ideas, are easy to switch to after the experimenter's point of view is understood, but the reverse is often not true. In other words, to enhance the usability of what you learn, you should pretend that you are a researcher, even if that is not your ultimate goal. As a researcher, one of the key skills you should be developing is to try, in advance, to think of all of the possible criticisms of your experiment that may arise from the reviewer of an article you write or the reader of an article you publish. This chapter discusses possible complaints about internal validity, external validity, construct validity, Type 1 error, and power. We are using \threats" to mean things that will reduce the impact of your study results on science, particularly those things that we have some control over. 191 192 CHAPTER 8. THREATS TO YOUR EXPERIMENT 8.1 Internal validity In a well-constructed experiment in its simplest form we manipulate variable X and observe the effects on variable Y. For example, outcome Y could be number of people who purchase a particular item in a store over a certain week, and X 8.1. INTERNAL VALIDITY 193 could be some characteristics of the display for that item, such as use of pictures of people of different \status" for an in-store advertisement (e.g., a celebrity vs. an unknown model). Internal validity is the degree to which we can appropriately conclude that the changes in X caused the changes in Y. The study of causality goes back thousands of years, but there has been a resur- gence of interest recently. For our purposes we can define causality as the state of nature in which an active change in one variable directly changes the probability distribution of another variable. It does not mean that a particular \treatment" is always followed by a particular outcome, but rather that some probability is changed, e.g. a higher outcome is more likely with a particular treatment compared to without. A few ideas about causality are worth thinking about now. First, association, which is equivalent to non-zero correlation (see section 3.6.1) in statistical terms, means that we observe that when one variable changes, another one tends to change. We cannot have causation without association, but just finding an association is not enought to justify a claim of causation. Association does not necessarily imply causation. If variables X and Y (e.g., the number of televisions (X) in various countries and the infant mortality rate (Y) of those countries) are found to be associated, then there are three basic possibilities. First X could be causing Y (televisions lead to more health awareness, which leads to better prenatal care) or Y could be causing X (high infant mortality leads to attraction of funds from richer countries, which leads to more televisions) or unknown factor Z could be causing both X and Y (higher wealth in a country leads to more televisions and more prenatal care clinics). It is worth memorizing these three cases, because they should always be considered when association is found in an observational study as opposed to a randomized experiment. (It is also possible that X and Y are related in more complicated ways including in large networks of variables with feedback loops.) Causation (\X causes Y") can be logically claimed if X and Y are associated, and X precedes Y, and no plausible alternative explanations can be found, particularly those of the form \X just happens to vary along with some real cause of changes in Y" (called confounding). Returning to the advertisement example, one stupid thing to do is to place all of the high status pictures in only the wealthiest neighborhoods or the largest stores, 194 CHAPTER 8. THREATS TO YOUR EXPERIMENT while the low status pictures are only shown in impoverished neighborhoods or those with smaller stores. In that case a higher average number of items purchased for the stores with high status ads may be either due to the effect of socio-economic status or store size or perceived status of the ad. When more than one thing is different on average between the groups to be compared, the problem is called confounding and confounding is a fatal threat to internal validity. Notice that the definition of confounding mentions “different on average". This is because it is practically impossible to have no differences between the subjects in different groups (beyond the differences in treatment). So our realistic goal is to have no difference on average. For example if we are studying both males and females, we would like the gender ratio to be the same in each treatment group. For the store example, we want the average pre-treatment total sales to be the same in each treatment group. And we want the distance from competitors to be the same, and the socio-economic status (SES) of the neighborhood, and the racial makeup, and the age distribution of the neighborhood, etc., etc. Even worse, we want all of the unmeasured variables, both those that we thought of and those we didn't think of, to be similar in each treatment group. The sine qua non of internal validity is random assignment of treatment to experimental units (different stores in our ad example). Random treatment assignment (also called randomization) is usually the best way to assure that all of the potential confounding variables are equal on average (also called balanced) among the treatment groups. Non-random assignment will usually lead to either consciously or unconsciously unbalanced groups. If one or a few variables, such as gender or SES, are known to be critical factors affecting outcome, a good alternative is block randomization, in which randomization among treatments is performed separately for each level of the critical (non-manipulated) explanatory factor. This helps to assure that the level of this explanatory factor is balanced (not confounded) across the levels of the treatment variable. In current practice randomization is normally done using computerized random number generators. Ideally all subjects are identified before the experiment begins and assigned numbers from 1 to N (the total number of subjects), and then a computer's random number generator is used to assign treatments to the subjects via these numbers. For block randomization this can be done separately for each block. If all subjects cannot be identified before the experiment begins, some way must be devised to assure that each subject has an equal chance of getting each treatment (if equal assignment is desired). One way to do this is as follows. If 8.1. INTERNAL VALIDITY 195 there are k levels of treatment, then collect the subjects until k (or 2k or 3k, etc) are available, then use the computer to randomly assign treatments among the available subjects. It is also acceptable to have the computer individually generate a random number from 1 to k for each subject, but it must be assured that the subject and/or researcher cannot re-run the process if they don't like the assignment. Confounding can occur because we purposefully, but stupidly, design our experiment such that two or more things differ at once, or because we assign treatments non-randomly, or because the randomization \failed". As an example of designed confounding, consider the treatments \drug plus psychotherapy" vs. \placebo" for treating depression. If a difference is found, then we will not know whether the success of the treatment is due to the drug, the psychotherapy or the combination. If no difference is found, then that may be due to the effect of drug canceling out the effect of the psychotherapy. If the drug and the psychotherapy are known to individually help patients with depression and we really do want to study the combination, it would probably better to have a study with the three treatment arms of drug, psychotherapy, and combination (with or without the placebo), so that we could assess the specific important questions of whether drug adds a ben- efit to psychotherapy and vice versa. As another example, consider a test of the effects of a mixed herbal supplement on memory. Again, a success tells us that something in the mix helps memory, but a follow-up trial is needed to see if all of the components are necessary. And again we have the possibility that one component would cancel another out causing a \no effect” outcome when one component really is helpful. But we must also consider that the mix itself is effective while the individual components are not, so this might be a good experiment. In terms of non-random assignment of treatment, this should only be done when necessary, and it should be recognized that it strongly, often fatally, harms the internal validity of the experiment. If you assign treatment in some pseudo- random way, e.g. alternating treatment levels, you or the subjects may purposely or inadvertently introduce confounding factors into your experiment. Finally, it must be stated that although randomization cannot perfectly balance all possible explanatory factors, it is the best way to attempt this, particularly for unmeasured or unimagined factors that might affect the outcome. Although there is always a small chance that important factors are out of balance after random treatment assignment (i.e., failed randomization), the degree of imbalance is generally small, and gets smaller as the sample size gets larger.

Load more