cross sectional study & sampling method

CROSS-SECTIONAL STUDY & SAMPLING METHOD

AP. DR. MOHD HASNI JAAFAR (ENVIRONMENTAL PHYSICIAN &

TOXICOLOGIST)Dept. Of Community Health

UKMMC.

Classification of Study Design

Observational – Case study– Case series– Cross-sectional – Case-control– Cohort

Experimental– Clinical trial– Laboratory

Cross-sectional study

Observational / Prevalence study.Cut across the study population only

one time.– Examine sample only at one time.– To answer: what is happening now?

What is happening?

What is the male: female ratio in this hall?

How many participants had hepatitis B vaccination in this hall?

How many participants felt into sleep now?

Definition

A study design that examines the relationship / association between ONE disease or other health –related characteristics and other MANY variables in ONE defined population at ONE particular time.

NOT for cause-effect or risk factors!

Condition

Study a disease or health problem– Description of affected population.– Associated causal and protective factors

present.– Magnitude or severity for control and

prevention program evaluation (may change health policy).

Randomization in sampling method.

Population Description

Disease distribution– Who (age, gender, ethnic group,

social class, occupation)?– When?– Where?

Age

Age or certain age groups.– Extreme age: morbidity mortality– Age: cumulative exposure (diet,

smoking, occupation, behaviour).– Extreme age: immune / hormone

E.g.: Malaria according to age

StateNumber of Malaria according to age group (year)

Total< 1 1 – 4 5 – 9 10 - 14 > 15

Perlis

Kedah

Pulau Pinang

Perak

Selangor

Negeri Sembilan

Melaka

Johor

Pahang

Terengganu

Kelantan

0

2

0

43

13

10

0

1

11

35

59

0

83

0

377

88

56

16

17

139

398

561

1

137

1

561

73

45

18

30

186

542

665

5

151

1

485

70

47

17

16

117

312

499

25

384

99

1534

217

249

204

304

692

914

1901

31

757

101

3000

461

407

205

368

1145

2201

3685

Pen. Malaysia 174

(1.4%)1735

(13.9%)2259

(18.2%)1720

(13.9%)6523

(52.6%)12 411 (100%)

Gender

Male female.– Physical and hormonal differences.– Occupation, leisure activity, daily

tasks.– Different exposure leads to different

health risks.

E.g.: Malaria according to gender

StateMalaria cases according to gender

TotalMale Female

Perlis

Kedah

Pulau Pinang

Perak

Kelantan

Terengganu

Pahang

Selangor

Negeri Sembilan

Melaka

Johor

27

158

29

1305

1897

780

1228

415

531

28

465

10

90

7

741

1185

300

590

209

314

3

57

37

248

36

2046

3082

1080

1818

624

845

31

522

Pen. Malaysia 6863 (66.2%) 3506 (33.8%) 10 369 (100%)

Ethnic

Ethnicity.– Different exposure (occupation,

homes, culture).– Body immune system (genetic).– Behaviour and socioeconomic

status.

E.g.: Malaria according to groups of people

State

Case of malaria

TotalArmy Police Orang Asli

Rancangan Tanah Kampung

Perlis

Kedah

Pulau Pinang

Perak

Selangor

Negeri Sembilan

Melaka

Johor

Pahang

Terengganu

Kelantan

8

22

40

329

18

13

124

28

80

15

199

2

17

4

269

0

67

1

55

148

66

74

0

0

0

210

232

167

61

18

206

0

7

1

14

0

259

2

12

0

85

155

417

1313

20

704

57

1933

209

148

69

182

556

1703

2092

31

757

101

3000

461

407

255

368

1145

2201

3685

Semenanjung Malaysia 876

(7.1%)703

(5.7%)901

(7.2%)

2258

(18.2%)

7673 (61.8%)

12 411 (100%)

E.g.: Ethnicity according to health problems (row %)

Health problemsNumber of cases according to ethnicity

Malay (%) Chinese (%) India (%) Total (%)

Caries

Pediculosis capitis

Low body weight

Obesity

Skin infection

Blindness

99 (29.5)

36 (16.6)

24 (24.0)

17 (31.5)

13 (22.4)

10 (19.2)

76 (22.6)

37 (17.0)

6 (6.0)

4 (7.4)

7 (12.1)

11 (21.2)

161 (47.9)

144 (66.4)

70 (70.0)

33 (61.1)

38 (65.5)

31 (59.6)

336 (100.0)

217 (100.0)

100 (100.0)

54 (100.0)

58 (100.0)

52 (100.0)

E.g.: Health problems, according to ethnicity (column %)

Health problemsNumber of cases according to ethnicity

Malay (%) Chinese (%) India (%)

Caries

Pediculosis capitis

Low body weight

Obesity

Skin infection

Blindness

Total

99 (49.8)

36 (18.1)

24 (12.1)

17 (8.5)

13 (6.5)

10 (5.0)

199 (100.0)

76 (53.9)

37 (26.2)

6 (4.3)

4 (2.8)

7 (5.0)

11 (7.8)

141 (100.0)

161 (33.7)

144 (30.2)

70 (14.7)

33 (6.9)

38 (8.0)

31 (6.5)

477 (100.0)

Description

When?– Air pollution (haze episodes).– Industrial accidents (explosion, chemical

spill).– Disaster (earth quake, drought, flood).– Recession (infrastructure, budget,

programme).

Description

Where?– Housing area (urban / rural / hostel).– Local area (district / mukim / kampung).– Topography (mountain, river).– Political geography (nation / state

/mukim / kampung)

What data are available?

Questionnaire (demography, associated factors, severity) – need validation.

Medical records.Physical examination.Laboratory investigation (blood, urine,

ultrasound, rapid test kits, X-ray, ECG).

What related factors - confounders?

Predisposing factors– Premorbid health problem.

Enabling factors– Poor nutrition, income, medical care.

Precipitating factors– Excess exposure to other risks.

Reinforcing factors– Repeated exposure to the agents.

How?

Sampling methods and techniques– Multi stage.– Stratified random sampling.– Simple random sampling.– Purposive.– Convenience.

Statistical output

Descriptive result– Percentages, ratio, population rate.– Prevalence rate (cross-sectional study output) = P.

Analytical result– Chi-square.– t-test.– ANOVA.– Correlation.– Linear regression.

Prevalence Rate

Rate / percentage of the disease in a population at a point of time.

Prevalence rate =

No. of all cases

at a period in timeX 100

Total population during that time

Factors influence the prevalence rate

Increase – Addition of new events (+)– Immigration of cases (+)– Emigration of healthy persons (-)– Longer life span among cases (+)– Less death among cases (+)

Chi-square test

Observed valueObserved value Expected valueExpected value

Infection M F Total

tve

-ve

29

67

24

80

53

147

Total 96 104 200

Infection M F Total

tve

-ve

25.44

70.56

27.56

76.44

53

147

Total 96 104 200

2 = (29 – 25.44)2/25.44 + (24 – 27.56)2/27.56 + (67 – 70.56)2/70.56 + (80 – 76.44)2/76.44

= 1.303 (p > 0.05); the Nul Hipotesis not rejected.

SAMPLING METHOD

A STUDY ON PREVALENCE OF SECONDARY HYPERTENSION AND ITS ASSOCIATED FACTORS AMONG ADOLESCENCE IN MALAYSIA

Target population – all HPT pts

Sample Population

Multistage random sampling

Sample Frame

Sample Unit

Stratified random

Simple random

SamplingRandom

Multistage / cluster sampling

– States

Stratified random– Departments / wards

Simple random– Patient list

Non random

Quota sampling– Gender balance

Purposive sampling. Convenience sampling. Matched samples.