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st: Collapse w. Weights


From   "Eric G. Wruck" <ewruck@econalytics.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Collapse w. Weights
Date   Sun, 6 Nov 2005 18:18:02 -0500

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



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

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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