Now, as you remember, the primary goal of observation is to describe behavior, but it is in reality impossible to observe and describe all of a person's behavior. So we have to rely on observing samples of people's behavior. Doing so, we must decide whether the samples represent people's usual behavior. Thus, we will first take a brief look at how researchers select samples of behavior. Before conducting an observational study, researchers must make a number of important decisions, that's about when and where observations will be made. As I've said before, the researcher cannot observe all behavior. Only certain behaviors occurring at particular times, in specific settings can be observed. In other words, behavior must be sampled. In this lecture, I will briefly introduce two kinds of sampling,that is, time sampling and situation sampling. Now first, time sampling. Time sampling means that researchers choose various time intervals for their observation. Intervals may be selected systematically or randomly. Suppose we want to observe students' classroom behavior. Then in systematic time sampling, our observations might be made during five 20-minute periods, beginning every hour. The first observation period could begin at 9 am, the second at 10 am and so forth.