14.2: Univariate Analysis
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Univariate analysis—or analysis of a single variable—refers to a set of statistical techniques that can describe the general properties of one variable. Univariate statistics include: frequency distribution, central tendency, and dispersion. The frequency distribution of a variable is a summary of the frequency—or percentages—of individual values or ranges of values for that variable. For instance, we can measure how many times a sample of respondents attend religious services—as a gauge of their ‘religiosity’—using a categorical scale: never, once per year, several times per year, about once a month, several times per month, several times per week, and an optional category for ‘did not answer’. If we count the number or percentage of observations within each category—except ‘did not answer’ which is really a missing value rather than a category—and display it in the form of a table, as shown in Figure 14.1, what we have is a frequency distribution. This distribution can also be depicted in the form of a bar chart, as shown on the right panel of Figure 14.1, with the horizontal axis representing each category of that variable and the vertical axis representing the frequency or percentage of observations within each category.
With very large samples, where observations are independent and random, the frequency distribution tends to follow a plot that looks like a bell-shaped curve—a smoothed bar chart of the frequency distribution—similar to that shown in Figure 14.2. Here most observations are clustered toward the centre of the range of values, with fewer and fewer observations clustered toward the extreme ends of the range. Such a curve is called a normal distribution .
Central tendency is an estimate of the centre of a distribution of values. There are three major estimates of central tendency: mean, median, and mode. The arithmetic mean —often simply called the ‘mean’—is the simple average of all values in a given distribution. Consider a set of eight test scores: 15, 22, 21, 18, 36, 15, 25, and 15. The arithmetic mean of these values can be calculated using the sum divided by the number of values . In this example, the mean would be 20.875. Other types of means include geometric mean ( th root of the product of numbers in a distribution) and harmonic mean (the reciprocal of the arithmetic means of the reciprocal of each value in a distribution), but these means are not very popular for statistical analysis of social research data.The second measure of central tendency, the median , is the middle value within a range of values in a distribution. This is computed by sorting all values in a distribution in increasing order and selecting the middle value. In cases where an even number of values in a distribution means there are two middle values, the average of those two values represents the median. In the above example, the sorted values are: 15, 15, 15, 18, 22, 21, 25, and 36. The two middle values are 18 and 22, and hence the median is .
Lastly, the mode is the most frequently occurring value in a distribution of values. In the previous example, the most frequently occurring value is 15, which is the mode of the above set of test scores. Note that any value that is estimated from a sample, such as mean, median, mode, or any of the later estimates are called a statistic .
Dispersion refers to the way values are spread around the central tendency—for example, how tightly or how widely the values are clustered around the mean. Two common measures of dispersion are the range and standard deviation. The range is the difference between the highest and lowest values in a distribution. The range in our previous example is .
The range is particularly sensitive to the presence of outliers. For instance, if the highest value in the above distribution was 85, and the other vales remained the same, the range would be . Standard deviation —the second measure of dispersion—corrects for such outliers by using a formula that takes into account how close or how far each value is from the distribution mean:
where is the standard deviation, is the th observation (or value), is the arithmetic mean, is the total number of observations, and means summation across all observations. The square of the standard deviation is called the variance of a distribution. In a normally distributed frequency distribution, 68% of the observations lie within one standard deviation of the mean , 95% of the observations lie within two standard deviations , and 99.7% of the observations lie within three standard deviations , as shown in Figure 14.2.