R: Probability of coincidences

birthday {stats} | R Documentation

## Probability of coincidences

### Description

Computes answers to a generalised birthday paradox problem. `pbirthday` computes the probability of a coincidence and `qbirthday` computes the smallest number of observations needed to have at least a specified probability of coincidence.

### Usage

    qbirthday(prob = 0.5, classes = 365, coincident = 2)
    pbirthday(n, classes = 365, coincident = 2)

### Arguments

`classes` | How many distinct categories the people could fall into
`prob` | The desired probability of coincidence
`n` | The number of people
`coincident` | The number of people to fall in the same category

### Details

The birthday paradox is that a very small number of people, 23, suffices to have a 50–50 chance that two or more of them have the same birthday. This function generalises the calculation to probabilities other than 0.5, numbers of coincident events other than 2, and numbers of classes other than 365.

The formula used is approximate for `coincident > 2`. The approximation is very good for moderate values of `prob` but less good for very small probabilities.

### Value

`qbirthday` | Minimum number of people needed for a probability of at least `prob` that `k` or more of them have the same one out of `classes` equiprobable labels.
`pbirthday` | Probability of the specified coincidence.

### Note

Prior to R 2.14.0 the approximate formula was used even for `coincident = 2`.

### References

Diaconis, P. and Mosteller F. (1989) Methods for studying coincidences. J. American Statistical Association, 84, 853–861.

### Examples

    require(graphics)

    ## the standard version
    qbirthday() # 23
    ## probability of > 2 people with the same birthday
    pbirthday(23, coincident = 3)

    ## examples from Diaconis & Mosteller p. 858.
    ## 'coincidence' is that husband, wife, daughter all born on the 16th
    qbirthday(classes = 30, coincident = 3) # approximately 18
    qbirthday(coincident = 4)  # exact value 187
    qbirthday(coincident = 10) # exact value 1181

    ## same 4-digit PIN number
    qbirthday(classes = 10^4)

    ## 0.9 probability of three or more coincident birthdays
    qbirthday(coincident = 3, prob = 0.9)

    ## Chance of 4 or more coincident birthdays in 150 people
    pbirthday(150, coincident = 4)

    ## 100 or more coincident birthdays in 1000 people: very rare
    pbirthday(1000, coincident = 100)

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