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Experimental design involves the manipulation of an independent variable in a study to determine the effect on a dependent variable, which is the variable of interest. The group to which the independent variable is assigned is known as the experimental group; the group in which no manipulation occurs is known as the control group. For example if the Fly-By-Night Pharmaceutical Company wanted to test the XYZ Elixir for stopping the hiccups, researchers would give the XYZ Elixir to one group of hiccuping people and a placebo to another group of hiccuping people. Stopping the hiccups would be the dependent variable, administering XYZ Elixir would be the independent variable, the group receiving the XYZ Elixir would be the experimental group, and the group receiving the placebo would be the control group. Testing for differences between the experimental group and the control group allows conclusions to be drawn about the effectiveness of the Elixir.

By contrast, survey research is retrospective – the effects of the independent variables on dependent variables are recorded after they have occurred. In this instance the Fly-By-Night Pharmaceutical Company might use a survey instrument to determine if people who used the Elixir reported different results in the treatment of their hiccups than people who did not use the Elixir.

Measurements

Numbers can be used in at least three different ways, depending on the level of measurement. When numbers are used in any fashion, data is created. Numbers may have identity, rank, and additivity. Series of numbers can be used (1) to categorize at the nominal level of measurement, (2) to rank or order at the ordinal level of measurement, and (3) to score at the interval level of measurement

Definition: Measurement is the assignment of numbers to observations.

Counting

The number of observations in a category is known as the frequency. A frequency distribution is a tabular display that displays how many individuals possess each value of a particular variable.

Percentages are used to compare groups of unequal size on an equitable basis. The common base for percentage is 100. Regardless of the number being compared in two or more categories, the base of comparison is 100 or more accurately 100% is the whole for each category.

Proportions have a base or total of 1.0. Proportions are always parts of something and can never exceed the total, which is 1.0. Proportions are frequently exchanged for probabilities.

Ratios describe rates and relationships and are fractions. The base, as with a proportion, is 1.0. Proportions are restricted to the relationship of a part to the total; ratios are helpful as index numbers. For example, IQ is an index number of the rate of general mental growth to chronological age.