Offwell Woodland & Wildlife Trust

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Processing & Interpreting Raw Data
Exercise Answers 5 and 6PreviousSection 1 (autumn)Section 2 (spring)Section 3 (autumn)Section 1 (autumn)Section 2 (spring)Section 3 (autumn)Section 4 (spring)Section 4 (spring)A test is required which will test the difference between two sets of data. The t-Test is one such statistical test. However, this test should not be used on small sets of data, which are not normally distributed. Most of the Bluebell data is not normally distributed, but is skewed (see below). Strip 1Strip 2Strip 3Strip 4(undivided)TopBottomTopBottomTopBottom% cover ClassNo of Samples0 - 103131341011 - 202452121 - 3013112131 - 401141 - 5023151 - 6021161 - 7011171 - 8011181 - 901191 - 100Another such test is the Mann-Whitney U Test. This would be more suitable as it is not dependent on data being normally distributed (it is 'distribution-free'). This test is also suitable for small sets of data ( 6 to 20 measurements). It is not suitable for sample sizes of greater than 20. (For the workings of a Mann-Whitney U Test carried out on the data click here)Median values for the Bluebell % cover data are calculated below for comparison with the means.Data sets ranked in order, with comparisons of mean and median valuesAutumn CutSpring cutAutumn CutSpring CutQuadrat No.Strip 1Strip 2Strip 3Strip 4(undivided)TopBottomTopBottomTopBottom158802589031010100420101005201513063015201745152418501525197020271109020402112045412204561320456142549815255515162560301725606018406775198070208580Mean3218.147.928.324.41.5Overall34.52538.113Median22.517.54522.511.51.0Overall252042.51.5Because the data is not normally distributed, outlying values which are unusually high or low have a disproportionate influence on the mean. This can therefore disguise real differences between the data sets. In this case, the median is probably a better figure to represent the mid point of individual data sets.Answers Contents