TURF.METHODS

General Considerations

INPUT FILE. A TURF run is done on an input file that contains some numeric items to be analyzed. The number of analysis items (NV) can be from 1 to 210, but will often be 20 or 30 or so.

The file can contain thousands of cases.

CASE.WEIGHT. The file may also have a weight variable which provides a weight for each case. If not, a weight of one is assumed for each case.

SIZE. A combination size needs to be provided. This is the number of items to be examined in each pass over the data. Suppose NV is 30 and the size is 7. A pass over the data will be done for every different combination of 7 of the 30 items, causing 2,035,800 passes.

In each pass, the number of cases that have been reached by the current combination of items is counted. The goal is to identify the combination that reaches the largest number of the cases.

RESPONSE.WEIGHTS. The response values themselves can be used as weights that reflect the intensity of the response. Using this option causes the responses for each case to be placed in memory without any change.

When response.weights is not used, the responses for each case are stored in 0/1 form; 0 if the response was indeed zero, and 1 if the response was any value greater than zero. This takes much less memory space.

ITEM.WEIGHTS. The items themselves are assumed to be equally important. In other words, the default is for each of the NV items to have a weight of one in the reach scoring. Different weights can be provided for some or all of the items. These are read from a file associated with the ITEM.WEIGHTS option.

REACH.THRESHOLD. Finally, the reach threshold, which defaults to one, can be changed to 3, for example, by saying REACH.THRESHOLD 3. The threshold can be fractional, like 3.5.

How the reach scoring is done

A case is reached when its reach score equals or exceeds the reach threshold. Suppose we are scoring a case on a combination that consists of items V2, V5, V11 and V17. Remember, the responses are stored internally as 0 or 1 except when the RESPONSE.WEIGHTS option is in use. The reach score for a given case is:

V2 response times V2's item weight, plus V5 response times V5's item weight, plus V11 response times V11's item weight, plus V17 response times V17's item weight.

If that score equals or exceeds the reach threshold, the case's caseweight is added to the number of cases that have been reached for that combination. One is used when there is no CASE.WEIGHT variable.

When responses and items are unweighted and the threshold is one, a case will have been reached when it has a positive response on any item in the combination.

How the freq scoring is done

If the case.weight option is not in use, consider each case to have a weight of one. A case's freq score depends on the options in use.

1. No use of response.weights or item.weights: Count the positive responses within a case on the variables in the combination. Multiply that count by the case's weight.
   
   
2. Response weights are used, but not item weights: Sum the positive responses within a case on the variables in the combination. Multiply that sum by the case's weight.
   
   
3. Item weights are used, but not response weights: Sum the item weights for those variables in the combination that have a positive value. Multiply that sum by the case's weight.
   
   
4. Response weights and item weights are in use: Sum the item weight times the response value for those variables in the combination that have a positive value. Multiply that sum by the case's weight.