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A to Z of Excel Functions: the EXPONDIST Function

4 March 2019

Welcome back to our regular A to Z of Excel Functions blog.  Today we look at the EXPONDIST function.

 

The EXPONDIST function

In probability theory and statistics, the exponential distribution (also known as negative exponential distribution) is the probability distribution that describes the time between events in a Poisson point process.  That’s good if you know what that is!  This is a process in which events occur continuously and independently at a constant average rate.  It is a particular case of the probability distribution known as the gamma distribution.  It is also the continuous analogue of the geometric distribution, and it has the key property of being memoryless.  In addition to being used for the analysis of Poisson point processes, it is also found in various other contexts.

It should be noted that the exponential distribution is not the same as the class of exponential families of distributions, which is a large class of probability distributions that includes the exponential distribution as one of its members, but also includes the normal distribution, binomial distribution, gamma distribution, Poisson and many others.

So, back to Excel.  This function returns the exponential distribution.  You should use EXPONDIST to model the time between events, such as how long an automated bank teller (ATM) takes to deliver cash.  For example, you can use EXPONDIST to determine the probability that the process takes at most one minute.

Important: This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage.  Although this function is still available for backward compatibility, you should consider using the new functions from now on, because this function may not be available in future versions of Excel, according to Microsoft.

The EXPONDIST function employs the following syntax to operate:

EXPONDIST(x, lambda, cumulative)


The EXPONDIST function has the following arguments:

  • x: this is required and represents the value of the function
  • lambda: this is also required and denotes the parameter value
  • cumulative: again, required.  This is a logical value that indicates which form of the exponential function to provide.  If cumulative is TRUE, EXPONDIST returns the cumulative distribution function; if FALSE, it returns the probability density function.

It should be further noted that:

  • if x or lambda is nonnumeric, EXPONDIST returns the #VALUE! error value
  • if x < 0, EXPONDIST returns the #NUM! error value
  • if lambda ≤ 0, EXPONDIST returns the #NUM! error value
  • The equation for the probability density function is:
  • The equation for the cumulative distribution function is:

Please see my example below: 

We’ll continue our A to Z of Excel Functions soon.  Keep checking back – there’s a new blog post every business day.

 

A full page of the function articles can be found here

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