- On 25 Oct 2005 at 20:12:47, Paul Donatus (hi2u_uk.at.yahoo.com) sent the message

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hello

I have performed a study in which i need to check to see if the drug

administered gave a statistically significant difference between day

0 and day 7 . In order to do this i used SPSS to plot a Q-Q plot to

test the normaility of the distribution of the difference. On looking

at the SPSS graph i note that it plots observed values which i assume

it has taken from my data against expected values. Does anyone know

where these expected values of the normal distribution come from ?

I also note that differences of 0 are not plotted. Does anyone know

why this is ?

Paul - On 25 Oct 2005 at 17:06:50, Navdeep Coelho (Navdeep.Coelho.aaa.biovail.com) sent the message

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Dear Paul,

The normal QQ plot graphically compares the distribution of the

difference between day 0 and day 7 (x-axis) to the normal

distribution (y-axis). If correctly plotted, the plot should show

both the actual data (day 0 - day 7)and the expected data (which

comes from the normal distribution) allowing you to visually

determine whether data are normally distributed. The closeness of the

data points to the 45 degree line quantifies the degree of normality

or lack thereof.

Regards,

Nav

--

Nav Coelho, M.Sc.

Biostatistician, Pharmacokinetics and Statistics

Biovail Contract Research

Tel: (416) 752-3636 Ext. 369

Email: navdeep.coelho.-a-.biovail.com

URL: www.biovail-cro.com - On 26 Oct 2005 at 14:20:18, Paul Donatus (hi2u_uk.aaa.yahoo.com) sent the message

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hello Nav

Thanks for your reply . What i do not understand about these Q-Q

plots is how does SPSS calculate the expected value of the normal

distribution . Does it take my values and then predict what they

would be if they were normally distributed and then this forms the

basis of the 45 degree line ? - On 26 Oct 2005 at 08:10:40, zdebelja.aaa.inet.hr sent the message

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The following message was posted to: PharmPK

Hi there.

Kolmogorov-Smirnov (KS) and Lilliefors tests of normality are based

on calculation of theoretical

cumulative frequencies at each experimental point one provides, in

this particular case the

differences between two measurements. For such calculation

experimental average and standard

deviation are calculated. These values are used for calculation of

cumulative normal distribution at

each point. Maximal difference between experimental cumulative

frequency and theoretical cumulative

frequency represents KS or Lilliefors statistic. Adherence of

experimental data to any other

theoretical distribution could be analyzed this way.

I think that calculated values used in Q-Q plot are theoretical

cumulative frequencies. Still, I do

not use this package and therefore I can not be certain.

Hope this helps.

Zeljko Debeljak, PhD student

Osijek Clinical Hospital

CROATIA - On 26 Oct 2005 at 11:12:45, Navdeep Coelho (Navdeep.Coelho.aaa.biovail.com) sent the message

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Hi Paul,

You are correct as QQ plot plots the actual data along the y-axis,

and the values that the data would have if they were exactly the

percentiles of a normal curve. Therefore, if the data is

approximately like that of a bell curve, the line should reseemble

the 45 degree line.

Hope this was helpful!

Nav

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