pbc = point biserial correlation: the correlation of an
individual problem with the total test.

N = number of test-takers in the sample.

These are plots of the number of people getting a particular problem correct (normalized to N) per total-score bin. The bins are distributed as follows:

Adjusted Score bin size |
Number of scores |

-5.75 to -2 | 5 |

-1.75 to 2 | 17 |

2.25 to 6 | 21 |

6.25 to 10 | 19 |

10.25 to 14 | 33 |

14.25 to 18 | 33 |

18.25 to 22 | 32 |

22.25 to 26 | 66 |

26.25 to 30 | 52 |

30.25 to 34 | 37 |

34.25 to 38 | 25 |

38.25 to 42 | 20 |

Total | 360 |