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**Descriptive Statistics**

Advanced Higher Geography Descriptive Statistics 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 don’t 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. 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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**Task Answers Find the mean, median and mode of the following data.**

5 mins ©Microsoft Word clipart 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 Answers Mean 5 Median 4 Mode 4

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**Midpoint x frequency (fx)**

Groups of data Sometimes the data we collect are in group form. Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-4 2 6 12 5-9 7 84 10-14 15-19 17 5 85 20-24 22 Total n = 30 ∑(fx) = 265 Finding the mean is slightly more difficult. We use the midpoint of the group and multiply this by the frequency.

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**Midpoint x frequency (fx)**

Calculating the mean Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-4 2 6 12 5-9 7 84 10-14 15-19 17 5 85 20-24 22 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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**Midpoint x frequency (fx)**

Calculating the mode Slope Angle (°) Midpoint (x) Frequency (f) Midpoint x frequency (fx) 0-4 2 6 12 5-9 7 84 10-14 15-19 17 5 85 20-24 22 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. 10 mins

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Chapter Eleven A Primer for Descriptive Statistics.

Chapter Eleven A Primer for Descriptive Statistics.

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