PTBISERIAL= point-biserial (instead of point-measure) correlation coefficients = No

The correlation between on the observations on an item (or by a person) and the person raw scores (or item marginal scores) are crucial for evaluating whether the coding scheme and person responses accord with the requirement that "higher observations correspond to more of the latent variable" (and vice-versa).

PTBISERIAL=Y or E        Compute and report the point-biserial correlation coefficients, rpbis, for persons and items. This is the Pearson product-moment correlation between the scored responses (dichotomies and polytomies) and the corresponding total (marginal) scores excluding the scored responses to be correlated, indicated on reports by PTBSE. Extreme (perfect, maximum possible and zero, minimum possible) scores are included in the computation, but missing observations are omitted pairwise. In Rasch analysis, rpbis is a useful diagnostic indicator of data miscoding or item miskeying: negative or zero values indicate items or persons with response strings that contradict the variable. Positive values are less informative than INFIT and OUTFIT statistics. The Biserial correlation can be computed from the Point-biserial.

PTBISERIAL=ALL        Compute the correlation between the total (marginal) scores including all responses and the responses to the targeted item and person, indicated on reports as PTBSA.

PTBISERIAL=N (or PTBISERIAL=RPM or =M). Compute and report point-measure correlation coefficients, rpm or RPM, shown as PTMEA These are reported in Tables 14 and 18 for items and persons. They correlate an item's (or person's) responses with the measures of the encountered persons (or items). rpm maintains its meaning better than rpbis in the presence of missing observations. Extreme measures are included in the computation. Negative or zero values indicate response strings that contradict the variable. Since the point-biserial loses its meaning in the presence of missing data, specify PTBISERIAL=N when data are missing or CUTLO= or CUTHI= are specified.

               PTBISERIAL=NO

       PTBISERIAL=ALL

Example 3: Here are the point biserial options in the current Winsteps

ni=2

codes=01

name1=1

item1=1

ptbis=yes  ; exclude current observation

&end

END LABELS

01

10

-------------------------------------------------------------------------------------

|ENTRY    RAW                   MODEL|   INFIT  |  OUTFIT  |PTBSE|EXACT MATCH|      |

|NUMBER  SCORE  COUNT  MEASURE  S.E. |MNSQ  ZSTD|MNSQ  ZSTD|CORR.| OBS%  EXP%| ITEM |

|------------------------------------+----------+----------+-----+-----------+------|

|     1      1      2     .00    1.41|1.00    .0|1.00    .0|-1.00|100.0  50.0| I0001|

|     2      1      2     .00    1.41|1.00    .0|1.00    .0|-1.00|100.0  50.0| I0002|

|------------------------------------+----------+----------+-----+-----------+------|

and

ni=2

codes=01

name1=1

item1=1

ptbis=all   ; include current observation

&end

END LABELS

01

10

-------------------------------------------------------------------------------------

|ENTRY    RAW                   MODEL|   INFIT  |  OUTFIT  |PTBSA|EXACT MATCH|      |

|NUMBER  SCORE  COUNT  MEASURE  S.E. |MNSQ  ZSTD|MNSQ  ZSTD|CORR.| OBS%  EXP%| ITEM |

|------------------------------------+----------+----------+-----+-----------+------|

|     1      1      2     .00    1.41|1.00    .0|1.00    .0|  .00|100.0  50.0| I0001|

|     2      1      2     .00    1.41|1.00    .0|1.00    .0|  .00|100.0  50.0| I0002|