Simple Interactive Statistical Analysis
Bug
Discussion.
This bug report concerns a problem in the SISA Tables for windows program. The bug was fixed on 1-12-2000. Read the following section of the helpfile first, then read the comment.
Section from helpfile
Rounding. This option is only relevant for the (Fisher) exact procedures and the non-technically minded should accept the defaults. The option does not pertain to approximate procedures or the ordinal or agreement exact procedures. The option allows for the treatment of rounding errors. Somewhere in the exact procedures it is determined if the p-value of a generated table is smaller than the point probably of the table by the statement: if p-this table<=point-p+theta then {add p-this table to cumulated p-value’s}. theta is a parameter to catch rounding errors. Should you not include theta you run the risk that two tables with the same p-value are not seen as such because the left p-value might have been upwardly rounded or the right p-value downwardly. This rounding error is a general Windows problem and takes place in the 18th significant digit. It might also affect the 17th or 16th significant digit. Auto sets theta to point probability * 1E-15. Auto is the default setting. Off sets theta to 0. Manual allows you to specify a value for theta yourself. You would normally choose a value smaller than 1E-15.
Comment
Basically the rounding correction did not exist before 1-12-2000, so theta was set to zero. If you want to study the effect of the problem try the table 12 8 8 12 in the various Fisher exact procedures with rounding correction on and off. The problem seems to particularly occur with multiples of 12 and you might want to check your other software for this problem as well. If someone knows what causes the problem and strategies to prevent this problem occurring, please let us know.
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