introduction to spss RESEARCH METHODS & STATISTICS HANDBOOK PHẦN 7 pptx

61

must be extremely aware of issues such as the level of data used and characteristics of

the population, namely distributional assumptions.

Levels of Data

Data conforms to one of four levels:

 Nominal (categorical) - the value is either present or not

 Ordinal - the value is ranked relative to others

 Interval - the value is scored absolute to others

 Ratio - the value is scored absolute to others and to a meaningful zero

An example:

Consider three horses in a race. Coding the race times under a nominal level will tell

us if any particular horse won the race or not (e.g. Guttman‟s Folly did not win).

Coding under an ordinal level, we can tell where a given horse came in relative to the

others (e.g. Guttman‟s Folly came in second). Coding under an interval level, we

know where a given horse came absolute to the others (e.g. Guttman‟s Folly was 1.5

seconds faster than Galloping Galton, but 2.3 seconds slower than Cattell‟s Chance).

Coding under a ratio level we would know where a given horse came absolute to the

others and a meaningful common zero point to all of them (e.g. Guttman‟s Folly came

home in 67.5 seconds, Galloping Galton was 69.0 seconds, and Cattell‟s Chance was

65.2 seconds).

Sometimes we use dichotomies. This basically is a variable that can only take one of

two values, either present (1) or absent (0). Its level therefore is nominal.

Descriptive Statistics

Measures of Central Tendency

There are three measures that give an indication of the „average‟ value of a data set:

 Mode - this is the most common value in the data set (most appropriate for

nominal level data)

 Mean - this is the arithmetic average, the one most familiar to people (most

appropriate for interval and ratio level data)

 Median - this is the middle value in the data set (most appropriate for ordinal level

data)

As an example, the following are the numbers of children for seven families:

0 0 0 1 1 5 7

 The mode (most common value) is 0.

 The mean is calculated as 14 (sum of all seven scores) divided by seven (number

of cases), which equals 2