Cross-sectional designs

Surveys are designed to describe some set of variables as they exist at the time of data collection. Data are collected from a sample over a relatively short period of time, shown as T₁ in Figure 5.1. The design used for a survey is also called a cross-sectional design because the sample is carefully selected to represent the cross section of some population. Either quantitative or qualitative techniques can be used to collect survey data, although, in general, quantitative techniques are used. The frequently cited Sudan Fertility Survey was based on a survey or cross-sectional design.

The survey design can be expanded to include more than one sample, as we will show shortly as part of the discussion of seeking differences between groups with respect to some variable.

Trend designs

Designs for trend studies are used to measure changes that may have occurred in one or more variables. The common design for trend studies is illustrated by B in Figure 5.1. Data for the same variable are obtained on two or more occasions, as illustrated by T₁ for the time of the first set of data, T₂ for the time of the second set, and so on to T_n, where the subscript n represents the time of the last set of measurements for the variable. Trend studies also can be based on use of data collected in earlier surveys with data from a current survey, as represented by C in Figure 5.1. T₁, T₂ and T₁ represent data collected at three points in the past while T_n, in this case, would represent data collected in a survey, which would provide a measurement of a variable at the present time. All four measurements could be used to establish a trend in a variable from points in the past to the present time.

For a variable to show a change from one time to the next, new conditions have to arise that can have some effect on the variable. The length of time required for any change to occur will vary, depending on the variables being investigated and the prevailing social conditions. Time intervals may be as short as weeks or months or, more often, extend over many years. Knowledge of the variables under investigation will help in deciding whether sufficient time has passed for changes to occur. This knowledge has to be matched against the time periods for which data are available. When using official statistics, the researcher has no choice: One has to use whatever is reported.

T₁ represents the time of the first data collection; T₂ represents the second; and T₃ the third. The dots (...) represent additional possible measurements. T_n stands for the last time of data collection.

Figure 5.1. Illustration of time as an element in research designs