I am trying to run an ancova and am having trouble setting it up properly.
I have nearly 10,000 measurements of fish length, girth and stage of sexual
development. I am suspicious that the stage of development is affecting the
length (as they get full of eggs they get more round and are more difficult
to measure and measure shorter).
My data looks somethign like this:

Length

girth

stage

40

50

2

42

48

3

37

40

5

38

38

6

34

44

2

36

45

3

36

39

4

39

42

4

39

39

6

but now I am not quite sure what to do next.

I have tried this: fit<-lm(length ~ girth + stage)

summary(fit)

Residuals:
      Min        1Q    Median        3Q       Max
-12.18198  -1.59198  -0.06057   1.50504  17.56265

Coefficients:
               Estimate Std. Error t value Pr(>|t|)
(Intercept)    24.97431    0.29956   83.37   <2e-16 ***
data$girth      0.24616    0.00571   43.11   <2e-16 ***
data$stage    0.67371    0.02742   24.57   <2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Residual standard error: 2.388 on 9864 degrees of freedom
  (1101 observations deleted due to missingness)
Multiple R-squared: 0.1934,    Adjusted R-squared: 0.1933
F-statistic:  1183 on 2 and 9864 DF,  p-value: < 2.2e-16


but this has not told me anything about where the differences in length that
are attributable to sexual development lie. any suggestions?


Thank you
Jacob

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