计算泊松概率百分比 [英] Calculate poisson probability percentage

问题描述

当您在 Excel(或 OpenOffice Calc)中使用 POISSON 函数时，它需要两个参数:

When you use the POISSON function in Excel (or in OpenOffice Calc), it takes two arguments:

• 一个整数
• 平均"数字

并返回一个浮点数.

在 Python 中(我尝试过 RandomArray 和 NumPy)，它返回一个随机泊松数数组.我真正想要的是这个事件发生的百分比(它是一个常数，数组每次都有不同的数字 - 那么它是平均值吗?).

In Python (I tried RandomArray and NumPy) it returns an array of random poisson numbers. What I really want is the percentage that this event will occur (it is a constant number and the array has every time different numbers - so is it an average?).

例如:

print poisson(2.6,6)

返回[1 3 3 0 1 3](每次运行它都不一样).

returns [1 3 3 0 1 3] (and every time I run it, it's different).

我从 calc/excel 得到的数字是 3.19 (POISSON(6,2.16,0)*100).

The number I get from calc/excel is 3.19 (POISSON(6,2.16,0)*100).

我是否使用了蟒蛇的泊松错误(不是双关语！)还是我遗漏了什么?

Am I using the python's poisson wrong (no pun!) or am I missing something?

推荐答案

手工操作很容易，但这样做可能会溢出.您可以在循环中执行指数和阶乘以避免溢出:

It is easy to do by hand, but you can overflow doing it that way. You can do the exponent and factorial in a loop to avoid the overflow:

def poisson_probability(actual, mean):
    # naive:   math.exp(-mean) * mean**actual / factorial(actual)

    # iterative, to keep the components from getting too large or small:
    p = math.exp(-mean)
    for i in xrange(actual):
        p *= mean
        p /= i+1
    return p

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