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Descriptive Statistics Advanced Higher Geography Ollie Bray – Knox Academy, East Lothian

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Descriptive statistics include: Types of data Measures of central tendency Measures of dispersion

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Types of data (1) Nominal data: data that has names. eg: rock types (sedimentary, igneous or metamorphic). Ordinal data: data that can be placed in ascending or descending order. eg: settlement type (city, town, village & hamlet).

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Types of data (2) Interval data: data with no true zero. Very uncommon so dont worry about it. Ratio data: most numerical data.

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Central Tendency When you calculate the central tendency of a data set you calculate its average. The measurements used for calculating central tendency include the mean, the mode and the median.

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The Mean Calculating the mean is one of the commonly used statistics in geography. Calculating the mean is one of the commonly used statistics in geography. It is found by totalling the values for all observations (x) and dividing by the total number of observations (n). The formula for finding the mean is: Mean = x n

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The Median The median is the middle value when all of the data is placed in ascending / descending order. Where there are two middle values we take the average of these.

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The Mode The mode is the number that occurs the most often. Sometimes there are two (or more) modes. Where there are two modes the data is said to be bi-modal.

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Find the mean, median and mode of the following data. The weekly pocket money for 9 first year pupils was found to be: 3 – 12 – 4 – 6 – 1 – 4 – 2 – 5 – 8 Mean 5 Mode 4 Median 4 5 mins ©Microsoft Word clipart

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Groups of data Sometimes the data we collect are in group form. Finding the mean is slightly more difficult. We use the midpoint of the group and multiply this by the frequency. Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-42612 5-971284 10-1412784 15-1917585 20-242200 Total n = 30 (fx) = 265

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Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-42612 5-971284 10-1412784 15-1917585 20-242200 Total n = 30 (fx) = 265 The mean is: (fx)/n =265 / 30 =8.8 Which is in the 5 – 9 group

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Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-42612 5-971284 10-1412784 15-1917585 20-242200 Total n = 30 (fx) = 265 We cannot find the mode for grouped data but we can find the modal group. The modal group. The modal group is the group that occurs most frequently (ie: 5-9 group).

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Your turn Read page 25 – 29 of Geographical Measurements and Techniques: Statistical Awareness, LT Scotland, June 2000. Answer questions 1 & 2 from Task 4 in this book.

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