> I'm no R expert, but I think I can help with this one.
>
> In short, the problem is that you are trying to evaluate dnbinom(x) for x
> not an integer. Well, not you exactly: but the curve function, which
> automatically makes a mesh of rational numbers over which to graph
> dnbinom(x).  On my system, the call produces a funny graph (flat with spikes
> at the integers if I jiggle the parameters just right) and produces warnings
> of the form:
>
> " In dnbinom(x, 6, 0.3) : non-integer x = 1.040000 "
>
> which should make the underlying problem pretty clear to you.
>
> One simple work-around, depending on what you want is to call instead:
>
> curve(dnbinom(floor(x),6,0.3))  or curve(dnbinom(ceil(x),6,0.3)) or
> curve(dnbinom(round(x),6,0.3))
>
> which will round all the non-integer calls down/up/nearest to an integer;
> this will produce a graph that I believe gets at what you want.
>
> You'll note that in your code that does work, you only call dnbinom at the
> integers 1,2,...,50.
>
> Depending on how much probability theory you know, you should be careful of
> how you interpret the graph that lines(1:50, dnbinom(1:50, 6, 0.3))
> produces...there's a few knotty technical issues relating to the values of
> the PMF between integers and this graph solves them in one way (linear
> interpolation), but you have to engage the question for yourself before
> making too many inferences. Since this is a somewhat thorny issue, my
> recommendation is that you handle the plot call yourself so you know exactly
> what you are getting.
>
> Hope this helps and good luck getting started with R,
>
> Michael Weylandt
>
>
>
>
>
>
> On Wed, Jul 13, 2011 at 4:05 AM, 郝立亚 <qqjwl@163.com> wrote:
>
>> Dear everyone,
>>      I want to draw the density lines of the a negative binomial
>> distribution X~Neg(6,0.3). The code lines(1:50, dnbinom(1:50, 6, 0.3)) work,
>> but curve(dnbinom(x, 6, 0.3)) doesn't work, why?
>>  Thank you very much!
>>        [[alternative HTML version deleted]]
>>
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>
>

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