I have data that looks like

lake,loglength,logweight
1,2.369215857,1.929418926
1,2.426511261,2.230448921
1,2.434568904,2.298853076
1,2.437750563,2.298853076
1,2.442479769,2.230448921
1,2.445604203,2.356025857
...
102,2.722633923,3.310268367
102,2.781755375,3.502153893
102,2.836324116,3.683407299
102,2.802773725,3.583312152
102,2.790285164,3.546419267
102,2.806179974,3.599118565
102,2.716837723,3.316180099


I can regress log weight on log length simply enough, but how would I model the third quartile of log weights?  In other words, rather than finding a 2nd quartile (or 50th percentile) regression line, 

e.g., mod=lm(logweight~loglength)

can R find a 75th percentile line?  Further, since my data is lake>1, is there a way to run 3rd quartile regressions on each lake?  I would imagine that regressing each population would require some call of the subset function, but I cannot figure out how to call it.

Thanks in advance, 

SR 

Steven H. Ranney
Graduate Research Assistant (Ph.D)
USGS Montana Cooperative Fishery Research Unit
Montana State University
PO Box 173460
Bozeman, MT 59717-3460

phone: (406) 994-6643
fax:   (406) 994-7479


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