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Correlation

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  • 1. October 4, 2012 Scatter diagrams2. Correlation and lines of best fit. Next
  • 2. Explanation October 4, 2012 You can see from this scatter graph that the sunnier it is, the more ice creams tend to be sold. This is called a positive correlation. Relationship between ice creams sold and hours of sunshine. The points do not lie 500  exactly along a straight 400 line, but you can clearly  see the general trendice creams sold   300 that the further to the  right the point is, the 200   higher up the graph it is. 100   The closer the point are  to a straight line, the 0 0 2 4 6 8 hours of sunshine 10 12 better the correlation. More Next
  • 3. Explanation October 4, 2012There are different types of positive correlation.                        Perfect positive Strong positive Weak positivecorrelation. correlation. correlation. More Next
  • 4. Explanation October 4, 2012Sometimes there is no correlation.         More Next
  • 5. Example October 4, 2012AnHours calledsalesman incorrelation. 2 as12 7ice5This is of sunshinenegative11the same town umbrella a 3 6 2 0 7 thecream vansold increases, the other variable tends7in theAs Umbrellas noted6how many 9umbrellas he had sold to one variable 5 2 11 4 8 0 5same ten days.decrease. 12  10  umbrellas 8  sold  6    4  2  0  0 2 4 6 8 10 12 hours of sunshine More Next
  • 6. Explanation October 4, 2012There are different types of negative correlation.                       Perfect negative Strong negative Weak negativecorrelation. correlation. correlation. More Next
  • 7. Explanation October 4, 2012Each of these lines is called the line if best fit.It shows more clearly the relationship between the twovariables, and helps you predict other results.                You are only expected to draw the line of best fit ‘by eye’.You need to interpret the angle of the overall trend.Try to have roughly the same number of points on eachside of the line. More Next
  • 8. Explanation October 4, 2012It is easy to spot a bad line of best fit. Here are two.                 Here the slope is correct, Here the number of but the number of points on points on each side is each side is unbalanced. equal, but the angle is There has been a clearly wrong. temptation to link three points on a straight line. More Next
  • 9. Example October 4, 2012 This scatter graph now has a line of best fit. How many ice creams would you expect to be sold on a day with 9 hours of sunshine? About 350 Relationship between ice creams sold and hours of sunshine. 500  This is just an estimate. 400 The better the correlation,   the better the estimate.ice creams sold   300  Avoid making an estimate 200  too far beyond the range   of your graph. 100  For example, here, for 16  0 hours of sunshine. 0 2 4 6 8 10 12 hours of sunshine More Next End

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