Sample Size in Quantitative Surveys Research
When conducting survey market research, the goal is to infer from the sample what is likely to be true of the target universe. A sample provides data that can be observed or known. From this observed or known data, a researcher can estimate the degree to which an unknown value or parameter can be found in a target population.
Quantitative surveys research is based on the notion of a normal, symmetrical curve that represents, in the mind of the researcher, the target universe - the population about which the researcher must estimate rather than actually know parameters. A representative sample allows a researcher to calculate -- from the sample data -- an estimated range of values that are likely to include the unknown value or parameter that is of interest. This estimated range of values represents an area on the normal curve, and is generally expressed as a decimal or a percentage.
The Normal Curve and Probability
A normal, symmetrical curve is a visual expression of probability. Let's look at a simple heuristic: An activity at a science center lets a large number of balls fall between two acrylic sheets, one at a time. Every ball falls through the same opening at the top of the display, and then drops between any of the vertical, parallel dividers that separate the stacks of balls once they come to rest. After several hours, the balls have formed the shape of a normal curve. The curve changes a little bit as each newly introduced ball hits the mass of balls that arrived first. But overall, the symmetrical curve is evident and it occurred naturally, independent of any action by the Science Center observers or staff. The curved shape that the balls form reflects the probability that most of the balls will fall into the center and stay there. Fewer balls will make it into the far ends of the curve - some inevitably will, but they are few in number.
This normal curve is similar to the concept of a sample. Each time the display is emptied out and the balls once again are allowed to fall into the Galton box, the configuration of the stacks of balls will be only a little bit different. But over time, the shape of the curve will not change much and the pattern will hold true.