3.  Let’s look at the Salk Vaccine experiment we first looked at on Wednesday.  In 1954, 200,475 children were given a treatment of Salk vaccine injections while 201,229 were given a placebo. The children didn’t know whether they were getting the vaccine or the placebo. They were assigned randomly to the two groups, equivalent to flipping a coin. Among the vaccine group, 33 children later developed paralytic polio and in the placebo group, 115 later developed the condition.

4.  There are several characteristics of an experiment that make it a well designed experiment. We will look at several of those as it relates to the Salk vaccine experiment.

5.  Randomization: Used when subjects are assigned to different groups through a process of random selection.  The logic behind randomization is to use chance as a way to create two (or more) groups that are similar.

6.  Replication: the repetition of an experiment on more than one subject  Samples should be large enough so that the erratic behavior that is characteristic of very small samples will not disguise the true effects of different treatments  In another context, replication refers to being able to replicate or duplicate an experiment to confirm or verify the results

7.  Replication: the repetition of an experiment on more than one subject  Just using a large sample size doesn’t ensure that it is a good sample  Use a sample size that is large enough to let us see the true nature of any effects, and obtain the sample using an appropriate method, such as one base on randomness

8.  Blinding: a technique in which the subject doesn’t know whether he or she is receiving a treatment or a placebo.  Allows us to determine whether the treatment effect is significantly different from a placebo effect  The polio experiment was double-blinded which means that 1. The children being injected didn’t know whether they were getting the Salk vaccine or a placebo 2. The doctors who gave the injections and evaluated the results did not know either

9.  Controlling the effects of variables: Design and control the experiment so that you can understand the effects of the variable  Try to design the experiment so that confounding does not occur  Confounding occurs when you are not able to distinguish among the effects of different factors  Pages 32-33 in the textbook

10.  Summary: three very important considerations in the design of experiments are the following: 1. Use randomization to assign subjects to different groups 2. Use replication by repeating the experiment on enough subjects so that effects of treatments or other factors can be clearly seen 3. Control the effects of the variables by using such techniques as blinding and a completely randomized experimental design

11.  No matter how well you plan and execute the sample collection process, there will likely be errors in the results  Sampling error: difference between the sample result and the true population result; due to chance sample fluctuations  Nonsampling error: occurs when the data are incorrectly collected, recorded, or analyzed.  We will learn methods to analyze sampling error, but must exercise caution as to minimize nonsampling error