AG 2011 10 17 - Laserfiche WebLink

Home Browse Search

CABARRUS COUNTY 2012 APPRAISAL MANUAL NON PARAMETRIC STATISTICS This class of statistics is useful in that unlike many statistical tools, they do not depend on having normally distributed values to be meaningful. The most usable is the chi - squared statistic. It is simple and is very useful in testing a number of common questions or hypotheses which you pose formally or informally in appraising. Suppose, for instance, you have collected a set of observations of the sale parcels in an area and you wish to compare the distribution of these sales with the distribution of all parcels for the area to see if the distributions match up and will give you some assurance that the sales are comparable to the universe of all parcels. To do this let us assume you use a single method of classification, age, and restrict the discussion to only a single exterior wall type (a good discriminator). How do you proceed? First classify the sale parcels into groups of 5 years although the greater of lesser intervals could have been selected depending on our data. For example: TABLE OF ACTUAL FREQUENCIES FOR SALE PARCELS AGE (in years) FREQUENCY PERCENT OF INTERVAL IN NUMBER TOTAL 1 - 5 10 13.2 6 - 10 22 28.8 11 - 15 17 22.4 16 - 20 10 13.2 21 - 25 7 9.2 26 - 30 10 13.2 76 100.0% Then classify all parcels for the area into groups of a like interval used with the sale parcels. For example: TABLE OF ACTUAL FREQUENCIES FOR SALE PARCELS AGE (in years) FREQUENCY PERCENT OF INTERVAL IN NUMBER TOTAL 1 - 5 128 12.2 6 - 10 234 22.4 11 - 15 355 33.9 16 - 20 139 13.3 21 - 25 87 8.3 26 - 30 104 9.9 1,047 100.0% Cabarrus County — 2012 Revaluation STATISTICS & THE APPRAISAL PROCESS 10-10 8/9/11 Attachment number 11 G -6 Page 305