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Date:         Sun, 6 Mar 2011 05:54:51 -0800
Reply-To:     Bruce Weaver <bruce.weaver@hotmail.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Bruce Weaver <bruce.weaver@hotmail.com>
Subject:      Re: Question about calculation of % change
In-Reply-To:  <AANLkTi=ehbYb3YkPv23qPJBMVog_7y90B5ZUpATTCykA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8

This illustrates one of the main problems with percentages -- percentage of what is not always clearly stated.

Justin Carroll wrote: > > Hmm.... > > I think the change can be measured in the terms you are talking about, but > when you get to proportional statements of change you can't alter the > scale > (linearly transform so to speak) without altering the proportional > statements itself. > > For example a 40% increase from "1" would be 1.40 > > A 40% increase from the original value of 100 would result in 140. > > It's not the range that is critical for the computation but the actual > values from Value1 to Value2; change the numbers, change the percent > increase (or decrease) - regardless of the 'range'. > > The difference between 1 and 4 is 3.. (as you said), and the difference > between 100 and 103 is also 3, but the % increased is dependent on the > numbers of Value_1 and Value_2. In this case, going from a score of 1 to > score of 4 is a 300% increase. An increase from 100 to 102, the same > "range", but is only 2% increase. > > Here is a quick and dirty calculator for percentage increase I found > online > (and of course you can do the hand-calculations as well): > > http://www.marshu.com/articles/calculate-percentage-increase-decrease-percent-calculator.php > > Hand Calc: > > Value1 > Value2 > > Value2 - Value1 = Difference > > Difference / Value1 = % of original value and the % of increase/decreased. > > To recalculate Value2 you can do an old accounting trick where you take > the > original value * (1+percentage changed). So for example, if I had Value1 > = > 10, Value2 = 14, the difference would be 4, but 4 is .40 (4/10) of 10 > (Value1). So if I want to reverse this, to get Value2, I could take the > original value of 10 and multiple by 1.40 (1 + the % change) which gets me > a > value of 14 again. > > Maybe I missed the OP's question or I am just not understanding the > problem/question - my apologies if that is the case; I assume he is > interested in commenting on percentage increase or decrease from the > original value, not the difference/distance from Time1 to Time2. > > > J. R. Carroll > Researcher for Hurtz Labs > Instructor at California State University, Sacramento > Research Methods, Test Development, and Statistics > www.jrcresearch.net > Cell: 916 628-4204 > Email: jrcarroll@jrcresearch.net > jrcarroll@hurtzlab.com > jrc.csus@gmail.com > > > > On Sat, Mar 5, 2011 at 8:51 PM, Swank, Paul R wrote: > >> The range on a 1 – 4 scale is 3 not 2.5 and the range on a 0 – 3 scale is >> also 3, so a 1 unit change is 33% either way. >> >> >> >> Dr. Paul R. Swank, >> >> Professor and Director of Research >> >> Children's Learning Institute >> >> University of Texas Health Science Center-Houston >> >> >> >> *From:* SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] *On Behalf >> Of *Max Freund >> *Sent:* Saturday, March 05, 2011 7:58 PM >> *To:* SPSSX-L@LISTSERV.UGA.EDU >> *Subject:* Question about calculation of % change >> >> >> >> Hello all, >> >> >> >> This is an embarrassingly basic question and not really SPSS-related, but >> I've got myself confused about it so I'm hoping this brain trust can help >> straighten me out. >> >> >> >> I have a variable measured on a 1-4 Likert scale at Time 1 and Time 2. I >> want to compute the percent change from T1 to T2. Would it be more >> accurate >> to transpose the scores to a 0-3 scale before calculating the % change? >> >> >> For example, a change from T1=2.5 to T2=3.5 measured on a 1-4 scale would >> result in a 40% increase (1/2.5=.4). When transposed to a 1-3 scale, >> however, it results in a 67% increase (1/1.5=.67). So basically, I'm >> wondering if starting the scale at 1 is a false minimum. >> >> >> >> The variable in question is an average of several items rating different >> aspects of organizational capacity and management practice, with 1 being >> little to no capacity in that area and 4 indicating robust capacity. >> >> Strictly speaking I know it may not be conceptually meaningful to compute >> percent changes in Likert items anyway (this one is actually an aggregate >> of >> several items). However, this is for an evaluation of a project that had >> as >> one of its objectives an X% increase in participants' capacity scores. >> >> Any thoughts? >> >> >> >> Thanks, >> >> Max >> >> >> >> -- >> >> Max Freund, M.I.I.M. • max@lunafreund.com • (909) 632-1624 >> >> Partner, LF Leadership (www.lfleadership.com) >> >> Doctoral Student in Organizational Behavior, Claremont Graduate >> University >> (www.cgu.edu/sbos ) >> >> >> >> Want to book a meeting with me? Check my availability at >> http://tungle.me/maxfreund >> >> >> >

----- -- Bruce Weaver bweaver@lakeheadu.ca http://sites.google.com/a/lakeheadu.ca/bweaver/

"When all else fails, RTFM."

NOTE: My Hotmail account is not monitored regularly. To send me an e-mail, please use the address shown above.

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