Political Science 30 Political Inquiry Drawing a Sample

Nominal Variables Measures of Central Tendency: Measures of Dispersion Mode Measures of Dispersion None, but you can note how many different values the variable may take on (how many categories there are)

Ordinal Variables Measures of Central Tendency: Measures of Dispersion Mode Median Measures of Dispersion “Range” (i.e. the variables “strength of religious belief” ranges from “very low” to “very high”)

Interval Variables Measures of Central Tendency: Mode Mean Median Measures of Dispersion Standard Deviation and Variance Range

Ratio Variables Measures of Central Tendency: Measures of Dispersion Mode Mean Median Measures of Dispersion Standard Deviation and Variance Range

Measures of Dispersion (Of Observations, p. 155) The variance is a measure of how spread out cases are, calculated by: Compute the distance from each case to the mean, then square that distance. Find the sum of these squared distances, then divide it by N-1.

Measures of Dispersion (Of Observations, p. 156) The standard deviation is the square root of the variance

Drawing Samples in “Observational Studies” Sample vs. the Population How to Draw a Random Sample Don’t Confuse a Random Sample/Selection with Random Assignment

Sample vs. the Population An observational study simply observes cases, without attempting to impose a treatment and without requiring any quasi- or natural experimental design. Researchers can ask their cases questions in order to measure some variable. Most of the time, researchers look closely at a small sample of the overall population.

Sample vs. the Population A population is the entire group of cases about which you want information. A sample is a subset of the population which is used to gain information about the whole population. Population Sample

Sample vs. the Population A parameter is a number describing a population. It is a usually a mystery. A statistic is a number describing a sample. Statistics vary from sample to sample. If our sample is representative of the population, sample statistics will closely approximate population parameters.

How to Draw a Random Sample A simple random sample gives all members of the population an equal chance to be drawn into the sample. Draw names out of a hat, a really big hat Label every case in the population with a number, then draw some random numbers In a telephone poll, random digit dialing uses a random number generator to get even those with unlisted numbers.