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RE: st: repeated measures at two time points

From   "Zekveld, Cornelia" <>
To   "''" <>
Subject   RE: st: repeated measures at two time points
Date   Mon, 1 Sep 2003 16:49:21 +0100


This is just descriptive work and I'm not testing a hypothesis - once I have
this graph I hope to be able to get the half-life from the graph.

-----Original Message-----
From: Ernest Berkhout []
Sent: 01 September 2003 16:40
Subject: Re: st: repeated measures at two time points

At 17:12 1-9-2003, you wrote:
>I hope someone can give me some pointers.  I have data on a few individuals
>(n=12) and for each person I have two measurements.
>The first was made at the last time of exposure (the calendar date differs
>for each person)
>The second was made on the same calendar date.
>So the follow-up differs for each person, (mean (sd) follow-up 89 (108)
>months).  So follow-up varies a lot.
>I can graph these two-time points for all individuals: with the x-axis
>time since last(final) exposure and the actual measurement value on the
>This produces a graph with 12 straight lines all starting at time 0 and
>ending at different points.
>Now my question is  - how can I summarise these 12 "lines".  I need to get
>one curve/line.

What is your null hypothesis? Can't you just plot the _difference_ in 
measurement value against the _difference_ in days between measurement and 
follow-up? You get 12 points in which may seem to indicate some 
relationship between followup-time and measurement-difference. Although 
significance should be considered carefully with such a small sample, I

Ernest Berkhout
SEO Amsterdam Economics
University of Amsterdam

Room 3.08
Roetersstraat 29
1018 WB Amsterdam
The Netherlands

tel.:+ 31 20 525 1657
fax:+ 31 20 525 1686
A statistician: someone who insists
on being certain about uncertainty

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