3. Add the squares of the entries in column (2):

 12 + 52 + 122 + 172 + 212 + 272 + 333 + 382 = 4162

4. Add the products of the entries in column (2) times the entries

 in column (8):

 (1)0 + 5(-.231) + 12(-.574) + 17(-.816) + 21(.-.997) + 27(-.1.093)

 + 33(-1.509) + 38(-1.262) = -170.1

5. The parameter "b" equals the result in step 4 divided by the

 result in step 3:
 -170.1 4162 = -.04087

6. The equation is y = e-'04087t

7. Cases remaining can now be estimated for any time period between

 one and 38 months. For example, the estimated proportion of
 cases remaining after 24 months in the system would be calculated

 as follows:

 Y = 2.7182818-.04087(24)

 Y = 2.7182818-.98088

 Y = .375 or 37.5 percent

8. The time required for a firm to lose a given proportion of cases

 can also be calculated. The formula below will allow you to

 solve for t given Y:

 t = In Y
 b

 For example, to calculate the number of months it would take for
 half the cases to leave the system, take the natural log of .50,

 divid the resulting logarithm by -.04087, the estimated "b" value
 discussed above.
 t = -0.693147 = 16.96 months
 -0.04087