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Test and CI for Two Proportions [leg pain]
 
Test and CI for Two Proportions [leg pain]
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Revision as of 12:01, 22 October 2009

Disc Fragments

The dream of every clinical trial is to come up with something which is inexpensive, definitive and likely to result in media publicity. “Improved outcome after lumbar microdiscectomy in patients shown their excised disc fragments: a prospective, double blind, randomized, controlled trial” by M.J. Tait, et al here fulfills the desire.

According to local Twin Cities website with the heading, Seeing, it appears, is believing when it comes to back surgery: “British surgeons report that patients who underwent a surgical procedure (lumbar microdiscectomy) for back pain caused by a spinal disc tear (“slipped disc”) had better outcomes when they received fragments of their removed disc after the operation. That’s right. Simply taking home a souvenir of the operation in a pot of saline solution improved the patients’ recovery. They reported less leg and back pain, less leg weakness and less “pins and needles” sensations (paresthesia). They also took fewer pain medications after the surgery.”

The surgeons “said they decided to do the study for two main reasons: They knew that a patient’s anxiety and depression going into surgery for a spinal disc tear has a big impact on the recovery process. They had also noticed, anecdotally, that many of their patients who responded best to the surgery — and who seemed to experience the least anxiety and depression afterwards — were those who had been given their disc fragments.”

The abstract of the journal article notes low p-values to make their case “that presenting the removed disc material to patients after LMD improves patient outcome.”:

Lumbar microdiscectomy (LMD) is a commonly performed neurosurgical procedure. We set up a prospective, double blind, randomised, controlled trial to test the hypothesis that presenting the removed disc material to patients after LMD improves patient outcome. METHODS: Adult patients undergoing LMD for radiculopathy caused by a prolapsed intervertebral disc were randomised into one of two groups, termed experimental and control. Patients in the experimental group were given their removed disc fragments whereas patients in the control group were not. Patients were unaware of the trial hypothesis and investigators were blinded to patient group allocation. Outcome was assessed between 3 and 6 months after LMD. Primary outcome measures were the degree of improvement in sciatica and back pain reported by the patients. Secondary outcome measures were the degree of improvement in leg weakness, paraesthesia, numbness, walking distance and use of analgesia reported by the patients. RESULTS: Data from 38 patients in the experimental group and 36 patients in the control group were analysed. The two groups were matched for age, sex and preoperative symptoms. More patients in the experimental compared with the control group reported improvements in leg pain (91.5 vs 80.4%; p<0.05), back pain (86.1 vs 75.0%; p<0.05), limb weakness (90.5 vs 56.3%; p<0.02), paraesthesia (88 vs 61.9%; p<0.05) and reduced analgesic use (92.1 vs 69.4%; p<0.02) than preoperatively. CONCLUSION: Presentation of excised disc fragments is a cheap and effective way to improve outcome after LMD.

The entire paper is only three pages in length and so its calculations can be checked. Below are the calculation results for the three secondary outcomes for which the paper claims statistical significance:

1. Improved Leg Weakness--the paper states that the p-value is less that .02. Minitab shows that the p-Fisher’s exact test is .024.

T valuefrom est and CI for Two Proportions [leg weakness]


Sample
X
N
Sample p
1
9
16
0.562500
2
19
21
0.904762

Difference = p (1) - p (2)
Estimate for difference: -0.342262
95% CI for difference: (-0.615844, -0.0686795)
Test for difference = 0 (vs not = 0): Z = -2.45 P-Value = 0.014

Fisher's exact test: P-Value = 0.024

2. Parathaesia--The paper states that the p-value is less that .05. Minitab shows that the p-value is from Fisher’s exact test is .08.

Test and CI for Two Proportions [parathaesia]


Sample
X
N
Sample p
1
22
25
0.562500
2
13
21
0.619048


Difference = p (1) - p (2) Estimate for difference: 0.260952 95% CI for difference: (0.0173021, 0.504603) Test for difference = 0 (vs not = 0): Z = 2.10 P-Value = 0.036

Fisher's exact test: P-Value = 0.080

3. Reduced Analgesic Use--The paper states that the p-value is less that .02. Minitab shows that the p-value from Fisher’s exact test is .017.

"""Test and CI for Two Proportions"""


Sample
X
N
Sample p
1
9
16
0.562500
2
19
21
0.904762

Difference = p (1) - p (2) Estimate for difference: 0.226608 95% CI for difference: (0.0534229, 0.399793) Test for difference = 0 (vs not = 0): Z = 2.49 P-Value = 0.013

Fisher's exact test: P-Value = 0.017

The primary outcomes, leg pain and (low) back pain for the treatment vs. the control were not calculated in a similar manner to the way the secondary outcomes were. Instead of using a two-sample test of proportions, the results for “pain” were calculated by having five categories: “Much better,” Little better,” “Same,” “Little worse,” and “Much worse.” That is, an ordinal scale was employed. Because the accompanying graphs, Figure 1A and 1B in the paper, are not precise enough to determine the number in each category, a nonparametric calculation is hard to carry out.

Nevertheless, ignoring the breakdown into five categories, here are Minitab results for leg pain and back pain, respectively; note that the p-values are much different from the claimed <.05:

Test and CI for Two Proportions [leg pain]

Sample
X
N
Sample p
1
35
38
0.921053
2
29
36
0.805556

Difference = p (1) - p (2) Estimate for difference: 0.115497 95% CI for difference: (-0.0396318, 0.270626) Test for difference = 0 (vs not = 0): Z = 1.46 P-Value = 0.144

Fisher's exact test: P-Value = 0.185

Test and CI for Two Proportions [back pain] Sample X N Sample p 1 33 38 0.868421 2 27 36 0.750000

Sample
X
N
Sample p
1
9
16
0.562500
1
33
38
0.868421

Difference = p (1) - p (2) Estimate for difference: 0.118421 95% CI for difference: (-0.0592271, 0.296069) Test for difference = 0 (vs not = 0): Z = 1.31 P-Value = 0.191

Fisher's exact test: P-Value = 0.242

Discussion

1. Why might an individual report a better outcome because he was handed his disc fragment? Why might he feel worse?

2. Assuming that the p-values reported in the article are correct, what criticism might still remain?

3. A disc fragment is one form of excised body part. What other excised body part might have a similar positive result? What other excise body part might have a distinctly negative result?

4. This study took place in London, England. Why might patient reaction be different in, let us say, Asia or Africa?

Submitted by Paul Alper