Download Drawing a Sample

Drawing a Sample

Measures of Central
Tendency:
 Mode

Measures of
Dispersion
 None, but you can
note how many
different values the
variable may take on
(how many
categories there are)

Measures of Central
Tendency:
 Mode
 Median

Measures of
Dispersion
 “Range” (i.e. the
variables “strength of
religious belief”
ranges from “very
low” to “very high”)

Measures of Central
Tendency:

Measures of
Dispersion
 Mode
 Standard Deviation
 Mean
and Variance
 Range
 Median

Measures of Central
Tendency:

Measures of
Dispersion
 Mode
 Standard Deviation
 Mean
and Variance
 Range
 Median

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.
(X  X )

Variance 
i
N 1
2

The standard deviation is the square
root of the variance
s  ˆ 
(X  X )
i
N 1
2

Sample vs. the Population

How to Draw a Random Sample

Don’t Confuse a Random
Sample/Selection with Random
Assignment

An observational study simply observes
cases, without attempting to impose a
treatment and without requiring any quasior 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.
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


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.

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.