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What proportion of patients was successfully treated?


BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

There are seven questions at the end of this article. This assignment is worth 80 points. Place

 

answers in this table:

 

1.

 

2.

 

3.

 

4.

 

5.

 

6.

 

7.

 


 

BIOSTAT Case Study: Tests of Association for Categorical Data

 

LEARNING OBJECTIVES

 

At the completion of this Case Study, participants should be able to:

 

Compare two or more proportions

 

Calculate and interpret confidence intervals for proportions

 

Understand the impact of expected values on the choice of statistical test used to

 

compare proportions

 

Interpret the results of tests of association

 

Interpret logistic regression results.

 


 

This material was developed by the staff at the Global Tuberculosis Institute (GTBI), one

 

of four Regional Training and Medical Consultation Centers funded by the Centers for

 

Disease Control and Prevention. It is published for learning purposes only. Permission

 

to reprint excerpts from other sources was granted.

 

Case study author(s) name and position:

 

Marian R. Passannante, PhD

 

Associate Professor, University of Medicine & Dentistry of New Jersey, New Jersey

 

Medical School and School of Public Health

 

Epidemiologist, NJMS, GTBI

 

For further information please contact:

 

New Jersey Medical School Global Tuberculosis Institute (GTBI)

 

225 Warren Street P.O. Box 1709

 

Newark, NJ 07101-1709

 

or by phone at 973-972-0979

 


 

1

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

Suggested Citation: New Jersey Medical School Global Tuberculosis Institute. /Incorporating

 

Tuberculosis into Public Health Core Curriculum./ 2009: BIOSTATISTICS CASE STUDY 2: Tests of

 

Association for Categorical Data STUDENT Version 1.0.

 


 

Introduction

 

This exercise is based on the following study. Sections of this document have been reprinted

 

with permission of the journal.

 

Factors influencing the successful treatment of infectious pulmonary tuberculosis W-S.

 

Chung,*? Y-C. Chang,? M-C. Yang?, * Department of Internal Medicine, Hualien General

 

Hospital, Hualien, ? Institute of Health Care Int J Tuberc Lung Dis 11:59?64 ? 2007 The Union

 

The abstract states that ?(t)his study used a population-based?design. All PTB [pulmonary

 

TB] patients residing in southern Taiwan recorded in the tuberculosis registry from 1 January to

 

30 June 2003 were identified. Each patient?s medical record was requested from treating

 

hospitals and retrospectively reviewed for 15 months after the date PTB was confirmed.? 1

 

Following is the methods section of this article 1.

 

METHODS

 

We carried out a population-based medical record review in southern Taiwan, where

 

the only chest specialty hospital geared towards specialized thoracic disease care,

 

mainly for TB, is located. Hospitals and primary practitioners that provided TB care in

 

the same region can be used as comparative care providers. Study areas include

 

Chiayi County, Chiayi City, Tainan County and Tainan City. As mandated by law in

 

Taiwan, all suspected and confirmed TB cases must be reported in a timely manner to

 

the national computerized registry maintained by the Taiwan Center for Disease

 

Control (CDC). Reporting of cases has been encouraged and reinforced through the

 

implementation of a no-notification, no-reimbursement policy and a notification-for-fee

 

policy since 1997. 7 We requested data on all suspected and confirmed TB patients

 

residing in the studied areas and recorded in the registry for the period 1 January to

 

30 June 2003. The study team, including four registered nurses (each with a

 

minimum of 6 years? clinical experience), two head nurses (each with a minimum of 12

 

years? clinical experience) and one pulmonologist, had undergone a series of training

 

courses designed to ensure proper validation of data consistency. Site visits were

 

arranged to review the medical record of each patient, and the 15-month follow-up of

 

medical records after start of treatment was reviewed.

 

Health care institutions

 


 

2

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

Health care institutions that had ever reported cases in the study areas included the

 

chest hospital, two academic medical centers, 11 regional hospitals and 15 district

 

hospitals and primary practitioners (district hospitals and primary practitioners are

 

regarded as being at the same level in terms of TB treatment). In Taiwan, institutions

 

are classified by the government as follows: ?medical centers? are health care, training

 

and research facilities that house over 500 acute-care beds; ?regional hospitals? have

 

no fewer than 250 acute care beds and are staffed by physicians of various specialties

 

with the purpose of providing health care services to patients and training for

 

specialists; and ?district hospitals? provide primary health care services similar to those

 

offered by primary practitioners but with the added availability of in-patient care.

 

Infectious PTB

 

Infectious PTB is defined as sputum culture-confirmed disease caused by

 

Mycobacterium tuberculosis, or two sputum smear examinations positive for acid-fast

 

bacilli (AFB) or one positive sputum examination, radiological signs and a clinician?s

 

decision to treat.8

 

Directly observed treatment

 

For directly observed treatment (DOT), a health worker or other trained person who is

 

not a family member watches as the patient swallows anti-tuberculosis medicines for

 

at least the first 2 months of treatment.1 DOT thus shifts the responsibility for cure

 

from the patient to the health care system. In Taiwan, whether or not the patient is

 

receiving DOT, TB is treated using WHO-recommended regimens; the initial phase

 

consists of 2 months of isoniazid (H), ethambutol (E), rifampicin (R) and pyrazinamide

 

(Z), followed by a 4-month continuation phase consisting of H, E and R

 

(2HERZ/4HER).9,10

 

Treatment success

 

Treatment success is defined as a patient who has been cured or has received a

 

complete course of treatment. A cured case is defined as a PTB patient who has

 

finished treatment with a negative bacteriology result during and at the end of

 

treatment. A case recorded as completed treatment is defined as a PTB patient who

 

has finished treatment, but who has not met the criteria to be defined as a cure or a

 

failure.11,12

 

Ethical consideration

 

The study was approved by the Taiwan CDC. All staff members involved in the study

 

signed a statement of agreement to maintain patient confidentiality.

 

Data analysis

 

Bivariate analyses with 2 tests were used to compare differences in proportions of

 

dichotomous and categorical variables, which extracted potential predictors of

 

successful treatment. We then performed multivariate logistic regression analyses on

 

the potential predictors with P < 0.10 obtained from bivariate analyses. We

 

constructed a full model that included all the potential predictors identified through

 

3

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

bivariate analyses and then applied the forward substitution model building procedure

 

to construct a reduced model in which all the predictors were statistically significant.

 

Odds ratios (ORs) and 95% confidence intervals (CIs) of dichotomous and categorical

 

risk variables on the binary outcome variables were calculated. All analyses were

 

conducted using SPSS 10.0 software (SPSS Inc, Chicago, IL, USA), and all the tests

 

were performed at the two-tailed significance level of 0.05.

 

References that appear in the excerpt from this article:

 

1 World Health Organization. Tuberculosis Fact Sheet. Geneva,Switzerland: WHO.

 

http://www.who.int/mediacentre/factsheets/fs104/en/index.html Accessed August 2006.

 

7 Chiang C Y, Enarson D A, Yang S L, Suo J, Lin T P. The impactof National Health Insurance

 

on the notification of tuberculosis in Taiwan. Int J Tuberc Lung Dis 2002; 6: 974?979.

 

8 Migliori G B, Raviglione M C, Schaberg T, et al. Tuberculosis management in Europe. Task

 

Force of the European Respiratory Society, the World Health Organization and the

 

International Union Against Tuberculosis and Lung Disease, EuropeRegion. Eur Respir

 

J 1999; 14: 978?992.

 

9 National Tuberculosis and Lung Disease Research Institute/World Health Organization

 

Collaborating Centre for Tuberculosis.Report on the Second Meeting of National TB

 

Programme managers from Central and Eastern Europe and the former USSR.

 

Bulletin No 3. Warsaw, Poland: WHO Collaborating Centre for Tuberculosis, 1997: 1?

 

30.

 

10 American Thoracic Society/Centers for Disease Control and Prevention/Infectious

 

Diseases Society of America. Treatment of tuberculosis. Am J Respir Crit Care Med

 

2003; 167: 603?662.

 

11 World Health Organization. Global tuberculosis control. WHO Report 1999.

 

WHO/CDS/CPC/TB/99.259. Geneva, Switzerland: WHO, 1999.

 

12 Farah M G, Tverdal A, Steen T W, Heldal E, Brantsaeter A B, Bjune G. Treatment outcome

 

of new culture positive pulmonary tuberculosis in Norway. BMC Public Health 2005; 5:

 

14.735?739.

 


 

Table 1, on the next page, presents the characteristics of the 399 patients eligible for this

 

study.1

 


 

4

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

Question 1

 

5

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

What type of study design is described in the abstract? (10 pts)

 

a.

 

b.

 

c.

 

d.

 

e.

 


 

Observational

 

Case Control

 

Retrospective

 

Cross Sectional

 

a and c

 


 

Question 2

 

What proportion of patients was successfully treated? (10 pts)

 

Question 3

 

Calculate a 95% Confidence Interval (CI) for the true population proportion with successful

 

treatment. Hint: The SE of p is the square root of (pq)/n. (10 pts)

 

Upper limit CI = _____

 

Lower Limit CI = _____

 

Question 4

 

Which of the following is true with regard to the confidence interval computed in Question 3

 

above: (10 pts)

 

a. 95 times out of 100 one would expect a the sample of 399 taken from the same

 

population to have a proportion of successfully treated patients to be between the

 

upper and lower limits of the confidence interval computed in Question 2.

 

b. 95 times out of 100 one would expect a the sample of 399 taken from the same

 

population to have a proportion of successfully treated patients to be outside the upper

 

and lower limits of the confidence interval computed in Question 2.

 

c. 5 times out of 100 one would expect a the sample of 399 taken from the same

 

population to have a proportion of successfully treated patients to be outside the upper

 

and lower limits of the confidence interval computed in Question 2.

 

d. a and c.

 


 

6

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

Question 5 (15 pts)

 

Using the information from Table 1, construct a 3 x 2 table to test the association between

 

DOT status and successful treatment.

 

Observed

 

DOT

 

Yes

 

No

 

Unknown

 

Total

 


 

Treatment Success

 

Yes

 

No

 

Total

 

250

 

146

 

3

 

275

 

124

 

399

 


 

Generate the expected values for the empty cells below. Hint: the expected value for any

 

cell is the row total x column total divided by the grand (overall) total.

 

Expected

 

DOT

 

Yes

 

No

 

Unknown

 

Total

 


 

Treatment Success Values

 

Yes

 

No

 

Total

 

250

 

146

 

3

 

275

 

124

 

399

 


 

Question 6

 

Using the DOT status groups, generate the chi-squared test statistic, by hand, using a

 

calculator, or using a computer.

 

Alpha = 0.05

 

df = _____ (5 pts)

 

Critical value = _____ (5 pts)

 

Chhi-squared test statistic = _____ (5 pts)

 

Based on comparing the Chi square statistic to the critical value which of the following is

 

true? (5 pts)

 

a.

 

b.

 

c.

 

d.

 


 

Successful outcome is dependent on DOT status.

 

Successful outcome is independent of DOT status.

 

No conclusion can be made.

 

The Chi square test is invalid because of only 1 degree of freedom.

 


 

7

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

Multiple logistic regression analysis allows us to look at the impact of independent variables

 

(potential predictor variables) on a dichotomous outcome variable such as successful

 

treatment completion (yes/no) when controlling for other independent variables. Table 3

 

presents some of the results of the multiple logistic regression model. 1 The outcome is

 

successful treatment.

 


 

Note:

 

Other footnotes are intentionally excluded from this table.

 

One way to assess the importance of a potential predictor variable is to examine the odds

 

ratios (ORs) and associated 95% CIs that are estimated from the logistic regression model.

 

Question 7 (10 pts)

 

Which independent variables listed below is (are) positively significantly associated with

 

successful treatment?

 

a.

 

b.

 

c.

 

d.

 

e.

 


 

Institutions

 

Physician

 

DOT

 

CXR

 

a, b, and c.

 


 

8

 


 

BIOSTATISTICS CASE STUDY:

 

Tests of Association for Categorical Data

 


 

References

 

1. Chung,*? Y-C. Chang,? M-C. Yang?, * Department of Internal Medicine, Hualien General

 

Hospital, Hualien, ? Institute of Health Care Int J Tuberc Lung Dis 11:59?64 ? 2007 The

 

Union

 

2. Dawson, B and Trapp, R Basic &Clinical Biostatistics, 4th edition, Lange Basic Science,

 

2004 page 152.

 


 

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