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# geometric mean month by month

Dear All,

I have two columns in a matrix B.
B=[A, X];
X is a column vector that contains different daily observations.
A is a column vector that contains the number of the month (for example for January it has "1" for february "2" etc till December where it has "12").
Column X has daily data for January, Feb till december. Thus Column A has numbers that change according to the month number. For instance, for january, Column A will have thirty one "1" corresponding to 31 days in january and twenty nine "2" corresponding to 29 days in February until the end of the year then i will start again with january with "1" and february with "2" etc till december .

What I would like to do is to do the gemetric mean of the values of X for similar values of A. Basically, it is like if Iam doing a geometric mean month by month untill the end of my data series. How can I do that please? I am stuck in this. ANy help is much appreciated. Best

S

## Products

Or maybe simpler I could just pick the first and the last day of the month and do a geometric mean of all values in between. Any ideas on how I can do that? Thank you very much

Answer by Eli Duenisch on 23 May 2013
Edited by Azzi Abdelmalek on 23 May 2013

Use logical indexing to obtain indices belonging to the same month and calculate the geometric mean for each month:

if true
for i=1:12, res(i)=geomean(X(A==i)); end
end

If you have no statistics toolbox installed you also need to write a function geomean() that calculates the geometric mean.

Hi Eli Thank you for your answer. I am not looking to geomean all january values or all february values but each year seperately. Thank you

Andrei Bobrov on 25 May 2013

Answer by Andrei Bobrov on 23 May 2013
Edited by Andrei Bobrov on 25 May 2013
out = accumarray(B(:,2),B(:,1),[],@geomean);

if the second column of your data is the same as:

b = arrayfun(@(x)x*ones(randi(7),1),repmat((1:12)',3,1),'un',0);
b = cat(1,b{:});