Isn't it repeated measures? What did you feed into GPower?
On 27 October 2010 11:38, Davies, Nicola <[log in to unmask]> wrote:
> Thank you, Jeremy. I worked out that we would need 88 participants in the pre-change group and 88 in the post-change group, and so went for 100 in each. I worked this out according to the research design - tests of difference, etc. However, my boss has asked how these figures relate to the properties of the questionnaire being used - I have no idea how to answer that!!!
>
>
>
> ________________________________________
> From: Jeremy Miles [[log in to unmask]]
> Sent: 27 October 2010 19:06
> To: Davies, Nicola
> Cc: [log in to unmask]
> Subject: Re: Power Calculations and Statistical Tests
>
> For which analysis? If you are using two measures at each time
> instead of one, then you reduce measurement error, that reduces
> variance which increases effect size. But not by a great deal
> (probably - you can work it out but you need to make an assumption
> about the reliability of the test). So I would leave the power
> analysis the same. For the slope with a break, it's fairly complex,
> so I'd do the power for the simple one, and say it's going to be more
> than that.
>
> That's a standard approach for a power analysis for a complex analysis
> - say "I'm going to base the power analysis on a simple analysis, the
> complex analysis is probably better, and therefore has more power.
> That means, if anything, I'm underestimating power, which I would
> rather do than overestimate power".
>
> Also, keep in mind that the Devil's Dictionary of Statistics by
> Stephen Senn defines power analysis as "Guessing, disguised as
> mathematics".
>
> Jeremy
>
>
>
>
> On 27 October 2010 10:23, Davies, Nicola <[log in to unmask]> wrote:
>> Thanks Jeremy.
>>
>> In regard to power analysis when choosing how many participants to recruit, how is this best achieved? I have used GPower to estimate number of participants needed based on the research design. Should I be basing power some how on the questionnaires I will be using? If so, how do I do this?
>>
>> Thanks for your help,
>>
>> Nicola
>>
>>
>>
>> ________________________________________
>> From: Jeremy Miles [[log in to unmask]]
>> Sent: 26 October 2010 23:24
>> To: Davies, Nicola
>> Cc: [log in to unmask]
>> Subject: Re: Power Calculations and Statistical Tests
>>
>> if you don't expect a trend, I would average the four (or two)
>> pre-test scores, and the four (two) post-test scores, and then do a
>> repeated measures t-test.
>>
>> If you expect some sort of consistent trend that you need to remove
>> then you need to do something like a multilevel model.
>>
>> Jeremy
>>
>>
>> On 26 October 2010 14:44, Davies, Nicola <[log in to unmask]> wrote:
>>> Hi,
>>>
>>> Does anyone know which statistical test I would need for the following:
>>>
>>> I have two independent groups: One group is before the service redesign; The second group is after the service redesign. Both groups will provide data at 4 points in time over a 12-month period.
>>>
>>> I need to know the statistical test I will use in order to calculate how many participants I need per group (I am using Gpower to do this).
>>>
>>> I will be trying to determine differences between the groups.
>>>
>>> Thank you for your help,
>>>
>>> Nicola
>>>
>>
>>
>>
>> --
>> Jeremy Miles
>> Psychology Research Methods Wiki: www.researchmethodsinpsychology.com
>>
>
>
>
> --
> Jeremy Miles
> Psychology Research Methods Wiki: www.researchmethodsinpsychology.com
>
--
Jeremy Miles
Psychology Research Methods Wiki: www.researchmethodsinpsychology.com
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