Re: Some way to differentiate (1st order derivative)?

[<pl...@pi...>]

On Wed, 24 Oct 2007 19:20:12 +0200, Ethan Merritt  
<merritt@u.washington.edu> wrote:

> On Wednesday 24 October 2007 05:45, pl...@pi... wrote:
>>
>> if you want to post that as an example it should probably by plotting
>> against ($1+old_x)/2 to prevent a shift in the position of the  
>> derivative.
>>
>> BTW the running mean example also suffers from this defect as is clearly
>> seen in the plot that Ethan linked to. The running mean is offset 2.5  
>> data
>> points to the right.
>
> The running mean example is correct for the typical use case.
> Have a look in your local paper for the stock price reports,
> or the weather summaries, etc.
>
> In calculating a running average you typically do not known the value of
> future points, only of the points measured to date.
> So you necessarily average over the previous 5 points, as stated in the
> plot title.
>
> This is not the same as for local approximation of the derivative.
>

Yes the title does clearly state what is shown. I did not say it was  
incorrect, I said it was a defect.

Hand-wavy discussions about some "typical usage" are not much help in  
science and I certainly would not look to my local paper for an reference  
on the correct way to process data.

I posted that comment since it is a common pitfall in calculating a  
running mean which also applied to the derviative code snip.

Please dont take it the wrong way.

;)