st: Collapse w. Weights

["Eric G. Wruck" <ewruck@econalytics.com>]

I have mutual fund data on turnover & total net assets.  Below you'll 
see an example:


     +-----------------------+
     | year   turnover   tna |
     |-----------------------|
  1. | 1997         .5   200 |
  2. | 1997        .73   172 |
  3. | 1997        1.1    25 |
  4. | 1998        .61   255 |
  5. | 1998        .96   220 |
     |-----------------------|
  6. | 1998        .85    75 |
     +-----------------------+


I want to get an asset-weighted average turnover ratio as well as 
report median turnover.

I issue the command:

. collapse (mean) avgturn=turnover (median) medturn=turnover 
[aweight=tna], by(year)

& obtain

     +---------------------------+
     | year    avgturn   medturn |
     |---------------------------|
  1. | 1997   .6374307        .5 |
  2. | 1998   .7827273       .85 |
     +---------------------------+

The average turnover ratios are precisely what I want, but I don't 
understand what Stata is doing with the medians.  I thought the 
weights wouldn't enter into  the calculations & that I would obtain 
medians of 0.73 & 0.85 for 1997 & 1998.  However, for 1997 I get a 
median turnover of 0.5.  Would somebody kindly explain this to me?

Many thanks,

Eric



--

===================================================

       Eric G. Wruck
       Econalytics
       2535 Sherwood Road
       Columbus, OH  43209

       ph:      614.231.5034
       cell:    614.330.8846
       eFax:    614.573.6639
       eMail:   ewruck@econalytics.com
       website: http://www.econalytics.com

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