Frequencies and probabilities

When a sample (chapter 2) of beings or of observations is classified with respect to a quantitative or qualitative variate, several observations may be located in the same class or have the same value. The number of beings or of observations “falling” in that class or possessing that value is then called the frequency (or absolute frequency) of that class or of that value. Since the sample is usually spread over several classes, the sum of the frequencies of all classes is called the total frequency. The total frequency of a sample is thus synonymous with sample size. Table 3.1.1 shows how a sample of 76 male North American martens (Martes americana) is spread, distributed, with respect to the length of the skull measured in mm. Such a table is called a frequency table. Table 3.1.1 Frequency table of the skull length X in mm of male North American martens captured in 1955 in Montana (Jolicoeur, 1963b, 1963c) Length classes (mm) Absolute frequencies Relative frequencies 68 0 0.0000 69 0 0.0000 70 0 0.0000 71 0 0.0000 72 0 0.0000 73 1 0.0132 74 0 0.0000 75 2 0.0263 76 3 0.0395 77 7 0.0921 78 13 0.1711 79 20 0.2632 80 12 0.1579 81 12 0.1579 82 6 0.0789 Sums of frequencies 76 1.0001