What is the role of the test statistic? [I'm not talking about the p-value]
- Thread starter Magnetar
- Start date
- Tags normal distribution normality shapiro-wilk test statistic
http://en.wikipedia.org/wiki/Shapiro–Wilk_test
But really it sounds like you need to understand what a test statistic is in general.
But really it sounds like you need to understand what a test statistic is in general.
Dude, no offence but are you drunk? I can't figure out from your reply when is W too small (when the statistic level yields a p value below the alpha level or otherwise?) In every definition of Shapiro-wilk test they only talk about the p value and alpha, but what means the W value, why is it there in the SPSS output for what sake?
If you can't manage basic curtesy good luck on discussing this issue with other posters. SPSS prints the statistic because individuals want to know what it is; the specific value is not critical for analyzing the null just the resulting p value. There is no such thing as "too small" or "too large", it is that value relative to your hypothesis that generates a specific p value that causes the null to be rejected or not.
Alternately, some argue that p values (hypothesis tests) are invalid and that the effect size is what matters. What is large enough to make a difference is not an absolute value. It depends on the question, your assumptions, and the like.
Alternately, some argue that p values (hypothesis tests) are invalid and that the effect size is what matters. What is large enough to make a difference is not an absolute value. It depends on the question, your assumptions, and the like.
"The fact, that W cannot take a value bigger than 1 is an important property of the W statistic.
We will discuss later that a value of W near one is an incidence that the underlying sample is
actually normally distributed, while by getting a small value of W we tend to reject the null
hypothesis of a normal distribution."
Finally I 've found it, it seems guys here know nothing about it. Naukowcy za grosze.
We will discuss later that a value of W near one is an incidence that the underlying sample is
actually normally distributed, while by getting a small value of W we tend to reject the null
hypothesis of a normal distribution."
Finally I 've found it, it seems guys here know nothing about it. Naukowcy za grosze.
W is not too horrible to calculate (although for large sample sizes it's not necessarily a walk in the park...). But the sampling distribution of W isn't too nice. I don't know exactly how each program is calculating the p-value but W is constrained between 0 and 1 (actually there is a lower bound slightly higher than 0 that is dependent on the sample size but it's not pretty so I don't feel like typing it up) and under the null hypothesis W should be 'close' to 1. If the data isn't actually normal we expect W to be further from 1. This is by construction of W and if you don't want to look into the math or the original paper then you'll just have to take my word for it.
So the p-value is obtained by looking at the probability that W is less than or equal to the observed value of W. This can be computed through some approximation or possibly by simulation - it depends on how the program decided to implement it.
Why is the observed value of W included in SPSS output? Because it's calculated and some people might be interested in it. I can't provide any more information than that but I don't see why it shouldn't be provided.
I posted the wikipedia link because you just asked "what it means" and it talks about that slightly. You never really explained what you meant by that though and it's hard to read your mind. You don't seem interested in understanding what test statistics are actually used for so I figured I'd provide a link that gives some details and you might be able to answer some questions yourself since it wasn't clear to me exactly what you wanted.
Please try to be more appreciative of the people attempting to help you. Nobody attacked you and we've only been trying to help. If the help we attempted to provide wasn't good enough then I suggest you try to provide more details on what you're looking for instead of asking the members here if we're drunk. I don't mean any offense but one sign that you're not doing a good job of asking a question is that you're not getting responses that seem to be helpful. I would suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly.
So the p-value is obtained by looking at the probability that W is less than or equal to the observed value of W. This can be computed through some approximation or possibly by simulation - it depends on how the program decided to implement it.
Why is the observed value of W included in SPSS output? Because it's calculated and some people might be interested in it. I can't provide any more information than that but I don't see why it shouldn't be provided.
I posted the wikipedia link because you just asked "what it means" and it talks about that slightly. You never really explained what you meant by that though and it's hard to read your mind. You don't seem interested in understanding what test statistics are actually used for so I figured I'd provide a link that gives some details and you might be able to answer some questions yourself since it wasn't clear to me exactly what you wanted.
Please try to be more appreciative of the people attempting to help you. Nobody attacked you and we've only been trying to help. If the help we attempted to provide wasn't good enough then I suggest you try to provide more details on what you're looking for instead of asking the members here if we're drunk. I don't mean any offense but one sign that you're not doing a good job of asking a question is that you're not getting responses that seem to be helpful. I would suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly.
There was serious discussion on whether or not to ban you. I decided against it for the time being. I advise that if you wish to be a member of this community you realize that we are only trying to help but you need to put in work as well. We aren't paid to do this. It is not reasonable to expect answers immediately. If that is your goal you should probably have a statistician on retainer.
As an aside I would suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly. TheEcologist put it together and I think it's a very nice summary of what the community expects from those that asks questions and in turn what you can expect from the community.
I sincerely hope you stick around. We don't take kindly to unprovoked insults though and you did ride the line of being banned today. If you do decide to abide by the guidelines and stick around I think you'll see we have a fairly knowledgeable community here that is more than willing to provide any assistance they can.
As an aside I would suggest checking out this thread for some guidelines on smart posting behavior that can help you get answers that are better much more quickly. TheEcologist put it together and I think it's a very nice summary of what the community expects from those that asks questions and in turn what you can expect from the community.
I sincerely hope you stick around. We don't take kindly to unprovoked insults though and you did ride the line of being banned today. If you do decide to abide by the guidelines and stick around I think you'll see we have a fairly knowledgeable community here that is more than willing to provide any assistance they can.