[ww-bugs] Bug 4090: New: Small calculation error possibility

[bugzilla-daemon at webwork.maa.org]

http://bugs.webwork.maa.org/show_bug.cgi?id=4090

           Summary: Small calculation error possibility
           Product: Problem Libraries
           Version: unspecified
          Platform: PC
               URL: /opt/webwork/courses/MRA_207-Spring_2018/templates/MC_
                    local/MC207/setStatistics_Ch11SamplingDistributions/11
                    Stats_13_SamplingDistns.pg_with_problemSeed=1512
        OS/Version: Windows
            Status: NEW
          Severity: minor
          Priority: P5
         Component: Library (OPL)
        AssignedTo: gjennings at csudh.edu
        ReportedBy: travis at mc.edu
        Web browser ---
           version:


I think that there are two errors in this exercise. 

Expanding the interval by 0.05s should take place with the original units and
not wait until conversion to z-scores.  I think it should be:

$zscore1 = int(100*( ($x1-.005 - $mu) / $sigmaSD)) / 100;
$pval1 = uprob(abs($zscore1));    #NOTE: The area is above the zscore
$x2 = $mu + random(0.1,0.2,0.01);
$zscore2 = int(100*( ($x2 +.005 - $mu) / $sigmaSD)) / 100;

Also, I don't think that one should subsequently round the z-scores to three
decimal places. (The int part.) This causes problems when someone computes the
answer when using a standard graphing calculator or online calculators on the
original values. The exercise does not suggest that one must first convert to
z-scores and then keep three significant digits and therefore should not
enforce that requirement internally.

Also, the author wants students to "expand" the interval by 0.005 on each side.
Perhaps a Hint indicating such would be appropriate IMO.

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