[R] finding birth position

[Deepankar Basu]

Hi All,

I have data on the sequence of births for families with completed
fertility cycle (in a data frame); the relevant variables are called b1,
b2, b3, b4, b5, b6 and record the birth of the first, second, ..., sixth
child. So, 
b1=1 if the first birth is male, 
b1=2 if the first birth is female, 
and b1=NA if the family did not record any first birth. 

Similarly for b2, b3, b4, b5 and b6.

I want to record the positions of the male children within their
family's birth history. So, I was thinking of creating six variables
boy_1, boy_2, ..., boy_6. boy_1 would record the position of the first
boy, boy_2 would record the position of the second boy and so on till
boy_6. I want to assign a value of zero to boy_i if the family in
question did not have the i_th boy.

I am not sure how best to do this (i.e., whether to create variables as
I have suggested or do something else) and would appreciate any
suggestions. Later, I want to use the information on the position of the
male births to compute a likelihood function and do an MLE. 

Here is how my data frame would look:

b1 b2 b3 b4 b5 b6 
1 2 1 2 NA NA 
2 2 NA NA NA NA 
1 2 1 1 1 NA 
2 1 NA NA NA NA 
1 NA NA NA NA NA 
2 1 2 1 NA NA 

Thanks in advance.

Deepankar