Warm Up – Take out a ½ sheet of paper… Check Your Understanding (p. 249) ~ This is a QUIZ so work independently! You have 8 minutes!

CHAPTER 4 Designing Studies 4.2 Experiments

Blocking (No this isn't about football!) When a population consists of groups of individuals that are “similar within but different between,” a stratified random sample gives a better estimate than a simple random sample. This same logic applies in experiments. Blocking in experiments is similar to stratification in sampling. A block is a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments. In a randomized block design, the random assignment of experimental units to treatments is carried out separately within each block. Completely randomized designs are the simplest statistical designs for experiments. But just as with sampling, there are times when the simplest method doesn’t yield the most precise results.

More about Blocking… Blocking accounts for a source of variability, just like stratifying. This means that blocking is another form of control. Blocks should be chosen like strata: the units within the block should be similar, but different that the units in the other blocks. You should only block when you expect that the blocking variable is associated with the response variable. The example on the previous slide is an example of blocking (smart phone users vs. non-smart phone users)

Example A popcorn lover wants to know if it is better to use the “popcorn button” on her microwave or use the amount of time recommended on the bag of popcorn. To measure how well each method works, she will count the number of unpopped kernels remaining after popping. To obtain the experimental units, she goes to the store and buys 10 bags each of 4 different varieties of microwave popcorn (butter, cheese, natural, and kettle corn), for a total of 40 bags. Explain why a randomized block design might be preferable to a completely randomized design for this experiment. Outline a randomized block design for this experiment. Describe how you would carry out the random assignment required by your design.

Matched Pairs Design A common type of randomized block design for comparing two treatments is a matched pairs design. The idea is to create blocks by matching pairs of similar experimental units. A matched pairs design is a randomized blocked experiment in which each block consists of a matching pair of similar experimental units. Chance is used to determine which unit in each pair gets each treatment. Sometimes, a “pair” in a matched-pairs design consists of a single unit that receives both treatments. Since the order of the treatments can influence the response, chance is used to determine with treatment is applied first for each unit.

Comparing Chocolate Chip Cookies Alternate example in your notes…(page 16)

Experiments DISTINGUISH between an observational study and an experiment. EXPLAIN the concept of confounding. IDENTIFY the experimental units, explanatory and response variables, and treatments in an experiment. EXPLAIN the purpose of comparison, random assignment, control, and replication in an experiment. DESCRIBE a completely randomized design for an experiment. DESCRIBE the placebo effect and the purpose of blinding in an experiment. INTERPRET the meaning of statistically significant in the context of an experiment. EXPLAIN the purpose of blocking in an experiment. DESCRIBE a randomized block design or a matched pairs design for an experiment.