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>
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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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