How to ignore data

Inline below. -- Bert

On Mon, Dec 13, 2010 at 9:42 AM, Steve Sidney<sbsidney at mweb.co.za>  wrote:

Thanks for the questions.

1) The data represents micro-organism counts and a count of zero in  this
case is highly unlikely given the info we have; including the other
participants.

?? Censoring or an experimental failure? Big difference.

2) The data is submitted in duplicate and then a standardised sum and
difference is established and is used to calculate a Z-score which is used
as a measure of performance.

Z scores are usually inappropriate for count data, which are discrete
and tend to be skew.

Given both 1) and 2) it is necessary to exclude a raw count of zero (since
the log of 0 is meaningless) and a count of one (since the log of 1 of
course is zero).

False. Correct statement is: "Because I do not know the statistical
methodology necessary to handle such discrete data with 0 counts, I
exclude them."  You are confusing your ignorance of statistical
methodology with the need for spurious ad hoc treatments. 0 counts can
and should be handled by appropriate statistical methods (e.g.
possibly 0 inflated Poisson models via glm() or otherwise).

I guess one can think of these values as outliers and that is what I am
trying to exclude.

This is a wholly unscientific statement, I'm afraid.

There is ample evidence that such an approach is acceptable.

What evidence, pray tell? -- a prior culture of inappropriate
analyses, perhaps? I do not wish to engage in a debate about this,
but, again, all I can say is that the above statement is not
scientific. If I were consulting with you, I would say "Please show me
your 'evidence.' " But, of course, I am not, and won't.

None of this is to say that you aren't correct in all respects. It is
just that you have raised all my usual warning flags, so that I am
somewhat skeptical. But that's MY problem. This is the last I will say
on the matter, so feel free to get in the final word, as I will not
respond.

And I wish you success in your efforts.

-- Bert

Thanks for the interest
Steve

On 2010/12/13 06:47 PM, Stavros Macrakis wrote:

If you need to take the log of the values for your calculation, then
what does it mean that you have 0 values in the input?

And why do you need to exclude the 1 values?

Are you sure that a) you are doing the correct kind of analysis and b)
the analysis is correct if you exclude 0 and 1?

             -s

On Mon, Dec 13, 2010 at 10:38, Steve Sidney<sbsidney at mweb.co.za>    wrote:

Dear list

I have quite a small data set in which I need to have the following
values
ignored - not used when performing an analysis but they need to be
included
later in the report that I write.

Can anyone help with a suggestion as to how this can be accomplished

Values to be ignored

0 - zero and 1 this is in addition to NA (null)

The reason is that I need to use the log10 of the values when performing
the
calculation.

Currently I hand massage the data set, about a 100 values, of which less
than 5 to 10 are in this category.

The NA values are NOT the problem

What I was hoping was that I did not have to use a series of if and
ifelse
statements. Perhaps there is a more elegant solution.

Any ideas would be welcomed.

Regards
Steve