When we take a census, we attempt to collect data from:
every individual in the population.
A study of treatments for angina (pain due to low blood supply to the heart) compared bypass surgery, angioplasty, and use of drugs. The study looked at the medical records of thousands of angina patients whose doctors had chosen one of these treatments.
We can't conclude that drugs prolong life because this was an observational study.
A simple random sample (SRS) is:
a sample that gives every possible sample of the same size to be selected.
The most important advantage of experiments over observational studies is that:
experiments can give better evidence of causation.
Consider an experiment to investigate the effectiveness of different insecticides in controlling pests and their impact on the productivity of tomato plants. What is the best reason for randomly assigning treatment levels (spraying or not spraying) to the
Random assignment will tend to average out all the other uncontrolled factors such as soil fertility so that they are not confounded with the treatment effects.
A TV station wishes to obtain information on the TV viewing habits in its market area. The market area contains one city of population 170,000, another city of 70,000, and four towns of about 5000 inhabitants each. The station suspects that the viewing ha
A stratified sample from the cities and towns in the market area.
Bias in a sampling method is:
any systematic error that tends to occur in the same direction whenever you use this sampling method.
You wonder if TV ads are more effective when they are longer or repeated more often or both. So you design an experiment. You prepare 30-second and 60-second ads for the camera. Your subjects all watch the same TV program, but you assign them at random to
This is a completely randomized design with two explanatory variables (factors).
A researcher wishes to compare the effects of 2 fertilizers on the yield of soybeans. She has 20 plots of land available for the experiment, and she decides to use a matched pairs design with 10 pairs of plots. To carry out the random assignment for this
subjectively divide the 20 plots into 10 pairs (making the plots within a pair as similar as possible) and then, for each pair, flip a coin to assign the fertilizers to the 2 plots.
You want to know the opinions of American high school teachers on the issue of establishing a national proficiency test as a prerequisite for graduating high school. You obtain a list of all high school teachers belonging to the National Educations Associ
The results of this survey cannot be trusted due to voluntary response bias.