[R] Dunif and Punif

R. Michael Weylandt michael.weylandt at gmail.com
Mon Nov 7 18:09:23 CET 2011

```A point of clarification:

dDIST() sometimes gives a pmf, e.g., dpois(). But dunif() is a pdf.

Sorry for the typo.

Michael

On Mon, Nov 7, 2011 at 12:08 PM, R. Michael Weylandt
<michael.weylandt at gmail.com> wrote:
> In short, the unif() distribution corresponds to the continuous
> uniform distribution, not the discrete.
>
> Longer: dDIST() doesn't give a pmf so summing it isn't what you are
> looking for: it gives a pdf. For punif() consider P\{X <= 1\} when X
> is distributed on [1, 10]. Clearly this has probability zero because
> it can only occur for one out of uncountably many values -- though
> this notion should be made more precise using a little bit of measure
> theory.
>
> Michael
>
>
> On Mon, Nov 7, 2011 at 11:49 AM, michele donato
> <michele.donato at wayne.edu> wrote:
>> Hi,
>> I am trying to use dunif and runif
>> however, I have two problems:
>> if I do
>>
>> dunif(1:10, min=1, max=10)
>>
>> I get 10 values, which summed give me 1.1111
>> I understand that the probability is computed as f(x) = 1 / (max-min)
>> but in this case it looks wrong: I have 10 values, each one
>> equiprobable, and the probability for each one should be 0.1 and not
>> 0.11111 (which is, consistently with the definition, 1/9)
>>
>> It looks like one of the extremes is not considered in the computation
>> of the probability, but then it's assigned a probability anyway.
>>
>> Similar problem with punif.
>>
>> if I do
>> punif(1, min=1, max=10)
>> I get 0 as result, as if the lower extreme is not considered, which is
>> not consistent with the description where min <= x <= max
>> If the lower extreme is not considered because cdf(x) = p(X<x)   {and
>> not p(X<=x)} the problem stands in p(X<11) which should be the sum of
>> everything. ( P(1) + P(2) + ... + P(10) )
>>
>> What is happening here?
>>
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