how to find the mean if the class intervals are unequal??

like fr e- 0-10,10-30,30-35....

Direct method

Assumed mean method

Step deviation method

Consider the following example.

A class teacher has the following absentee record of 40 students of a class for the whole term. Find the mean number of days a student was absent.

No. of days | 0-6 | 6-10 | 10-14 | 14-20 | 20-28 | 28-38 | 38-40 |

No. of students | 11 | 10 | 7 | 4 | 4 | 3 | 1 |

Here is the solution.

CLASS INTERVAL | FREQUENCY (f_{i}) |
CLASS MARKS(x_{i}) |
f_{i}x_{i} |

0-6 | 11 | 3 | 33 |

6-10 | 10 | 8 | 80 |

10-14 | 7 | 12 | 84 |

14-20 | 4 | 17 | 68 |

20-28 | 4 | 24 | 96 |

28-38 | 3 | 33 | 99 |

38-40 | 1 | 39 | 39 |

Total | ∑ f_{i}_{ }= 40 |
∑ f_{i}_{ }x_{i}_{ }= 499 |

Mean = ∑ f

_{i}

_{ }x

_{i}

_{ }/ ∑ f

_{i}

_{ }= 499/40 = 12.475 $\approx $ 12.48

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