Date: Tue, 22 May 2007 00:52:42 +0000
Reply-To: rwwalker@optonline.net
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Bob Walker <rwwalker@optonline.net>
Subject: Re: Analysing change in proportion of species over time
In-Reply-To: <651616.71823.qm@web32006.mail.mud.yahoo.com>
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Hi Bethwell,
Couldn't you use multivariate GLM? Yes, you have repeated measures, but in his case there is a significant intervening variable, which is time. Time should, therefore, be treated as a factor in your design. The data looks to be:
DATA LIST FREE / YEAR (F4) SPECIES (A3) CONT ROT PCT_CONT PCT_ROT.
BEGIN DATA
1992,AB,440,701,0.0484,0.0842
1992,BE,110,165,0.0121,0.0198
1992,BI,308,218,0.0339,0.0262
1992,CP,764,1571,0.084,0.1886
1992,CV,296,428,0.0325,0.0514
1992,DM,927,681,0.1019,0.0818
1992,ES,114,131,0.0125,0.0157
1992,ESP,531,659,0.0584,0.0791
1992,FB,810,739,0.0891,0.0887
1992,HC,3825,1624,0.4206,0.195
1992,PN,42,37,0.0046,0.0044
1992,RN,135,47,0.0148,0.0056
1992,SI,177,73,0.0195,0.0088
1992,TB,247,559,0.0272,0.0671
1992,TT,348,694,0.0383,0.0833
1992,UM,21,3,0.0023,0.0004
1993,AB,1031,1270,0.0328,0.0464
1993,BE,8049,4673,0.2557,0.1706
1993,BI,184,318,0.0058,0.0116
1993,CP,680,1333,0.0216,0.0487
1993,CV,2163,1679,0.0687,0.0613
1993,DM,372,444,0.0118,0.0162
1993,ES,71,128,0.0023,0.0047
1993,ESP,709,474,0.0225,0.0173
1993,FB,3913,3946,0.1243,0.1441
1993,HC,4063,1925,0.1291,0.0703
1993,PN,4558,4891,0.1448,0.1786
1993,RN,408,367,0.013,0.0134
1993,SI,171,520,0.0054,0.019
1993,TB,4481,4803,0.1423,0.1754
1993,TT,160,406,0.0051,0.0148
1993,UM,467,209,0.0148,0.0076
END DATA.
* Why not run GLM / Multivariate as follows.
GLM PCT_CONT PCT_ROT BY SPECIES WITH YEAR
/METHOD = SSTYPE(3)
/INTERCEPT = EXCLUDE
/PLOT = PROFILE( SPECIES )
/EMMEANS = TABLES(OVERALL) WITH(YEAR=MEAN)
/EMMEANS = TABLES(SPECIES) WITH(YEAR=MEAN)
/PRINT = DESCRIPTIVE ETASQ OPOWER PARAMETER LOF
/CRITERIA = ALPHA(.05)
/DESIGN = YEAR SPECIES.
This should isolate the main + interaction effects between species and time. HTH,
Bob Walker
Surveys & Forecasts, LLC
www.safllc.com
----- Original Message -----
From: Bethwell Moyo
Date: Monday, May 21, 2007 5:23 pm
Subject: Analysing change in proportion of species over time
To: SPSSX-L@LISTSERV.UGA.EDU
> Dear all,
>
> I have data on number of certain species in a rotational
> grazed plot and also the number of same species in a continuosly
> grazed one collected for 8 years. The idea is to check whether
> there is a change in their proportion as year progressed. I have
> tried repeated measures, but was discouraged. A suggestion to
> compare proportions was put forward, and i don`t know how to
> test significant differences in proportions between the two
> plots. Part of the data looks like this:
> year Species Continuous
> Rotational Prop_Cont Prop_Rot 1992 AB 440 701 0.0484
> 0.0842 1992 BE 110 165 0.0121 0.0198 1992 BI 308
> 218 0.0339 0.0262 1992 CP 764 1571 0.0840 0.1886
> 1992 CV 296 428 0.0325 0.0514 1992 DM 927 681 0.1019
> 0.0818 1992 ES 114 131 0.0125 0.0157 1992 ESP 531
> 659 0.0584 0.0791 1992 FB 810 739 0.0891 0.0887
> 1992 HC 3825 1624 0.4206 0.1950 1992 PN 42 37 0.0046
> 0.0044 1992 RN 135 47 0.0148 0.0056 1992 SI 177
> 73 0.0195 0.0088 1992 TB 247 559 0.0272 0.0671 1992
> TT 348 694 0.0383 0.0833 1992 UM 21 3 0.0023 0.0004
> 1992 Total 9095 8330 1993 AB 1031 1270 0.0328
> 0.0464 1993 BE 8049 4673 0.2557 0.1706 1993 BI 184
> 318 0.0058 0.0116 1993 CP 680 1333 0.0216 0.0487
> 1993 CV 2163 1679 0.0687 0.0613 1993 DM 372 444
> 0.0118 0.0162 1993 ES 71 128
> 0.0023 0.0047 1993 ESP 709 474 0.0225 0.0173 1993
> FB 3913 3946 0.1243 0.1441 1993 HC 4063 1925 0.1291
> 0.0703 1993 PN 4558 4891 0.1448 0.1786 1993 RN 408
> 367 0.0130 0.0134 1993 SI 171 520 0.0054 0.0190
> 1993 TB 4481 4803 0.1423 0.1754 1993 TT 160 406
> 0.0051 0.0148 1993 UM 467 209 0.0148 0.0076 1993
> Total 31480 27386
>
>
> How do I show that for example there has been a signicant
> change in proportion of species AB, in 1993. I want a
> statistical method of dertemining that, especially using proportions.
>
> Thanx for the help
>
>
> ===================================================================
> It is only with the heart that one can see rightly. What is
> essential is invisible to the eye.
> Mr B Moyo
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Bob Walker
Surveys & Forecasts, LLC
www.safllc.com
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