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help for ^optfixn^                                                 (STB-58: sxd2)
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Optimal sampling design for 2-stage study with fixed second stage sample size
-------------------------------------------------------------------------

  	^optfixn^ depvar [indepvars] [^if^ exp] [^in^ range]  
	[^, first(^varlist^)^ ^n1(^vecname^)^ ^n2(^#^)^ 
	^var(^#^)^ ^coding(^#^)^


Options
-------

^first(^varlist^)^ specifies the first stage variables.

^n1(^vecname^)^ specifies the vector of first stage sample sizes for each
            stratum formed by different levels of dependent variable and first
            stage covariates. (See Description below for how to enter this
            vector.)

^n2(^#^)^       specifies the second stage sample sizes.

^var(^#^)^      specifies the position in the logistic regression model of the
            covariate whose variance is to be minimized (i.e. optimized). For
            example, in the simple model Y = b0 + b1X1 + b2X2, if we want to
            minimize the variance of X1, then ^var^ = 2.

^coding(^#^)^   is a logical flag; the default of 0 (FALSE) means that prior to
            calling the ^optfixn^ function, you have run the
            ^coding^ function (help @coding@ for details) to create the
            vector ^grp_yz^, which contains the distinct groups (strata)  
            formed by the different levels of response (Y) and first stage
            covariates (Z). If you have not run ^coding^ and you call the
            ^optfixn^ function with ^coding=1^, the ^grp_yz^ vector will be
             created within the ^optfixn^ function, but it is imperative that
            the vector vecname is provided to ^optfixn^ in the correct
            order! For this reason, we strongly suggest that any call to
            ^optfixn^ is preceded by a call to ^coding^.


Description
------------

The ^optfixn^ function calculates the optimal sampling fraction in each of the
strata at the second stage, for situations where first stage data is available
and the total number of second-stage observations has been decided. Before
running the ^optfixn^ function you should run the ^coding^ function, (see help
@coding@), to see the order in which you must supply the vector of first stage
sample sizes in the various strata.


Examples
--------

As an illustration, we provide the pilot data described in Table 1
of Reilly (Am. J. Epi, 143, 92-100, 1996), and the following 
commands produce the sampling fractions of Table 3 in that paper.

 . ^use pilotcas^

 . ^coding mort sex^
     
 . ^matrix fstsamp=(6666, 1228, 144, 58)'^ 
	
 . ^optfixn mort sex age,first(sex) n1(fstsamp) n2(1000) var(3)^


The ^coding^ command identifies the strata formed by levels of
the dependent variable (mort) and first stage covariate (sex),
and the matrix "fstsamp" contains the first stage sample sizes 
in the correct order. 

					 
Author
------

      	Marie Reilly, Dept. of Epidemiology & Public Health
	and
	Agus Salim, Dept. of Statistics,

	University College Cork, Cork, Ireland
	marie.reilly@@ucc.ie


Also see
--------

    STB:  STB-58: sxd2
 Manual:  [R] logit, glm
On-line:  help for @logit@, @glm@, @coding@, @optbud@, @optprec@