MSC Project

jill

New Member
#1
Iam doing a project looking at two ways to scan a knee and comparing them against surgery they had prior to the scans however it is only in patients who have had surgery is this a conventient sample ? and what tests can i use. I have to wrie a bit in my dissertation plan ??? :mad:
 
#2
Jill,

Are you slelcting x number of patients from N patients who underwent surgery? If the selection is random I think it's a random sample.

The scans before and after surgery for each patient is dependent. You can use a paired t-test to analyze the data, (or a Wilcoxon Signed Rank Test if the population is not normally distributed).

Let me know if you have further questions.
 

JohnM

TS Contributor
#4
The tests to be used actually depend on what kind of data you're looking at - in other words, how are you quantitatively comparing the scans?

That will determine what type of test is appropriate.
 

jill

New Member
#5
I have a provisional ethical acceptance now however Iam having trouble with my sample size. It is a paied design. I have a limited number of patients ie.40 and am finding this hard to justify. Any suggestions ??

Jill
 
#6
Jill,

How much statistical power and how large an effect size are you looking for? Is the sample of n=40 representative of your study population? How are the samples collected and are you doing any randomization? Sorry for all these questions, but I need to know these information in order to justify any sample size. I actually have many more questions. :)
 

JohnM

TS Contributor
#7
Woops! Jin snuck in there while I was editing / away from my computer!:D

Jill,

Just so I get your study straight in my head:

40 patients who have had knee surgery
20 will be assigned at random to 1 of 2 different scan methods
you want to do a before-after comparison for each of the two groups of 20

Well, yes, it is a convenience sample, but so are lots of biomedical studies - I challenge anyone to find me a single biomedical study that is a purely random sample of the population...

Anyway, a few questions:
- could you get a sample (20 - 40) of similar aged people who have not had surgery? if so, why? I guess I need to understand your study more.
- what's the problem with n=40? especially with a paired design, any statistical test with n=20 and matched pairs should be a pretty powerful test....
- with the scans - exactly what are you "measuring" or evaluating - i.e., what is your dependent variable?
 
#8
John: Yes convenient samples are common in biomedical studies, and n=40 could very well be sufficient.

Jill: Please provide more details regarding your study design. ie how patients are paired and assigned treatment. etc.
 

jill

New Member
#9
The sample n=40 is limited due to the number of patients who have had previous surgery within the hospital. The sample is collected randomly from the computer system storing the images and reports

From n=40 All patients will have both types of scan following which thay will have a 'gold standard' op with a camera to determine the definative result

Each of the scans has a yes tear or no tear answer which will be the variables

I hypothesize MR arthrography is significantly more accurate at detecting specific meniscal pathology than plain MR alone in patients who have had previous meniscal surgery.
 

JohnM

TS Contributor
#11
Jill,

If you were a statistician, you wouldn't be here, and a lot of other people wouldn't be here, and Jin would never get this site to where he wants it to be:)

Anyway-

Now I see where the sample size issue comes into play. It looks like you're basically comparing the proportion (percentage) of accurate diagnoses between scan method, and you really need a large sample size for proportions.

If you think that one method will be definitively better than the other, then maybe you will still be able to detect a statistical difference, however, if you think that the two methods will be somewhat close, then there may be a problem with statistical power.

Do you have any idea, approximately, what the accuracy rates may be for the two methods, or what the difference may be?

With n=40, and worst case accuracy is around 50% (i.e., no better than flipping a coin), then the standard error of a proportion will be:

SEp = sqrt(p*q/n)
where p=rate of correct diagnoses
q = rate of incorrect diagnoses = 1-p
n = sample size

so, in this example, SEp = sqrt(.5*.5/40) = .079 or approx 8%

Now for a 95% confidence level, you'll need to multiply this % by 1.96, so that brings it up to .155 or 15.5%
- in order to detect a difference between methods, the difference would need to be at least 15-16%, but this is a "worst case" estimate

If the actual diagnosis accuracy rates are higher, then the 15-16% would drop off a bit.....
 
#12
Cheers John

The accuracy rate without dye have been reported to be between 50-80% and with dye the accuracy increased to 88%–92%.

Jill
 
#14
what about sensitivity and specificity i think the problem is on one of the papers i have read the expected prevelence was 68% from a past paper without dye had a sensitivity of 86%, specificity of 67%, PPV of 83%, NPV of 71%, with dye sensitivity 90%, specificity 78%, PPV 90%, NPV 78%
Jill
 

JohnM

TS Contributor
#15
jill,

how many participants were in that study?

you may not have enough participants to analyze sens, spec, PPV, NPV for each method, because those %'s are pretty close, although the method w/dye appears to be slightly better in all 4 respects.

however, if you want to just make a basic statement about diagnosis accuracy, then you should be OK.

JohnM
 
#16
there were 104 patients in the other study. just worried with a 68% expected prevelence (n=40) that only leaves around 13 patients who wont have tears ??? I just need to some how write something that will keep the ethics comitee happy using some sort of sample size formula statistical power etc.. any ideas
Jill
 

JohnM

TS Contributor
#17
Jill,

If you go to the Examples section on this site, I have a post on Sample Size Determination.

Look at the one regarding estimation of a percentage, with a specified margin of error, at a certain level of confidence.

Good luck.

John
 
#18
Hi John

So if i want to use the past average of around 65% without dye to estimate the actual percentage to within +/- 3 percentage points, at a 95% confidence

n >= ((p*q)/d^2) * Z^2

n >= ((0.65* 0.35)/(0.03^2)) * 1.96^2

n >= (.228/.0009) * 3.8416

n >= 97

Therefore i would need 97 patients. Is this what you ment for me to do ??

Jill
 

JohnM

TS Contributor
#19
Yes, but check your math - you should get n >= 972

Like I mentioned before, if you're willing to go with a simple comparison of accuracy -

no dye --> 50-80%, or avg 65%
with dye --> 88-92% or avg 90%

then you could set the margin of error higher, possibly as high as 15%, and reduce n to 39
 
#20
Ok

Got the correct result (i dont believe it iam so stressed i cant even do simple maths now!!!!) If i just want to use a simple comparison of accuracies at 65% and 90% is it possible to set this out in the mathamatical form to get n=39 as i cant achieve this ?? sorry for being such a hassle

Jill