Crossectional studies 2

DESCRIPTIVE AND CROSSECTIONAL STUDIES

DR. F D E MTANGO

OBJECTIVES

• AT THE END OF THIS LECTURE THE STUDENT WILL BE ABLE TO:

• OUTLINE THE STEPS IN THE SCIENTIFIC METHOD• DIFERENCIATE BETWEEN OBSERVATIONAL AND

EXPERIMENTAL STUDIES• KNOW THE TYPES OF STUDIES FOR FORMULATION AND

FOR TESTING OF AN EPIDEMIOLOGIC HYPOTHESIS• KNOW THE SCOPE AND TYPES OF DESCRIPTIVE

STUDIES• DETERMINE THE ADVANTAGES AND DISADVANTAGES

OF DESCRIPTIVE AND CROSSECTIONAL STUDIES• GIVE AN EXAMPLE OF AT LEAST ONE DESCRIPTIVE AND

ONE ANALYTICAL STUDY

STEPS IN THE SCIENTIFIC METHOD

IDENTIFICATION OF A RESEARCH PROBLEM

DESCRIBING THE PROBLEM (DISEASE) NO CONTROL

GROUP

APPLICATION OF CONTROL STRATEGIES FORMULATION OF INTO NATIONAL PROGRAMMES HYPOTHESIS

FIELD TESTING OF STRATEGIES TESTING OF HYPOTHESIS (THE’RE’S CONTROL GROUP)

OBSERVATIONAL STUDIES:» CROSSECTIONAL,» CASE CONTROL,» COHORT STUDIES

EXPERIMENTAL studies

FORMULATION OF CONTROL STRATEGIES

EARLY STEPS IN THE SCIENTIFIC METHOD

• STEP I; IDENTIFICATION OF RESEARCH PROBLEM

• STEP II: (WE ARE HERE)DESCRIPTIVE STUDIES (SURVEYS : HEALTH, MORBIDITY & UTILIZATION SURVEYS – INCIDENCE AND PREVALENCE SURVEYS, CASE STUDIES: FAMILY & COMMUNITY

• STEP III: FORMULATION OF HYPOTHESIS

MIDDLE STEPS IN THE SCIENTIFIC METHOD

STEP IV: TESTING OF HYPOTHESIS

a) OBSERVATIONAL ANALYTICAL STUDIES:

CROSSECTIONAL, CASE CONTROL, COHORT STUDIES

b) EXPERIMENTAL STUDIES:

RANDOMIZED CONTROLLED FIELD TRIALS, COMMUNITY INTERVENTIONS, CLINICAL TRIALS

LATER STEPS IN THE SCIENTIFIC METHOD

• STEP V: FORMULATION OF CONTROL STRATEGIES

• STEP VI: FIELD TESTING OF COTROL STRATEGIES

• STEP VII: APPLICATION

OF CONTROL STRATEGIES

INTO NATIONAL DISEASE CONTROL PROGRAMMES

USES of descriptive studies

• Need for describing new diseases or epidemics

• Need to formulate new hypothesis

• Baseline studies for control of diseases or control programmes

• Evaluation of control programmes or management practices

DESCRIPTIVE STUDIES

DESCRIPTIVE STUDIES USUALLY EXAMINE THE HEALTH STATUS OF THE COMMUNITY BASED ON ROUTINELY AVAILABLE DATA , E.G. MORBIDITY AND MORTALITY STATISTICS OR THROUGH SURVEYS (EG/ THE NATIONAL CENSUS).

THE PATTERN OF DISEASES IS DESCRIBED ACCORDING TO PERSONS INVOLVED, THEIR AGES, SEXES, OCUPATIONS, RACES, EDUCATION, PLACES AND TIME.

NO HYPOTHESIS IS BEING TESTED

Types of descriptive studies

1.Correlation studies :These are studies that describe disease or other

health problem in relation to some factor, eg. per capita food consumption, per capita cigarette consumption, annual imports of alcohol, mean annual rainfall, etc. The disease can be measured in terms of disease rates (prevalence or incidence) or mortality rates (e.g. infant mortality rates). Two variables (e.g. one on exposure and the other on disease rates) are correlated using the correlation coefficient (r),

Type of descriptive studies..2

2.Case reports:

A case report describes the experience of a single patient or a group of patients with similar diagnosis. A case report can document an unusual medical occurrence and can, therefore represent the first clue in the identifications of new disease or adverse effects of exposure : e.g. drugs

Types of descriptive studies.3

3. Case series: This is a collection individual case reports, which may occur within a fairly short period of time. e.g. the cases of Pneumocystis carinii pneumonia (PCP) which were described at the beginning of the AIDS epidemic in young homosexuals as a new finding, never described previously. Previous to these cases, PCP , had only been seen amongst elderly patients suffering from debilitating diseases like cancer. The observation of a series of Kaposi sarcoma cases occurring amongst young people was seen after the appearance of the HIV epidemic.

Types of descriptive in relation to time

Descriptive studies can be

• crossectional: data collected at one point in time,

• prospective : data collected in the future

• Retrospective: data collected in the past

Descriptive studies- person(who is affected?)

• The kind of persons afflicted by the disease; what are their characteristics? e.g. age, sex, race, socio-economic status, genetic constitution, immunology status etc. These are then related to those of the population to which such persons belong, i.e. census data to determine if there is any predisposition to the disease.

Descriptive studies- place(where are the patients ?)

• The kind of places where diseased individuals are found or geographical placement, altitude, latitude, climate, vegetation etc. An example of the use of place in descriptive studies is found in the studies of Burkitt’s lymphoma in Africa.

• The influence of place in determining disease occurrence can also be studied by using migrant populations, eg. occurrence of cancer by site for Japanese of different migratory status as shown in the table below)

Migrant studies of Cancer (ca rate per 100,000 popolation).

Site Jap/Japan Japs/Am. Sons/Am/Jap

Sto 8.4 3.8 2.8

Liver 4.1 2.7 2.2

Colon 0.2 0.4 0.9

Descriptive studies-time(when?)

• Time (when ?) : Does the disease have any change in time (trend)? Many infectious diseases occur during certain periods of the year (seasonal distribution). This can be related to the development of breeding sites for disease vectors, hence increased disease transmission.

• Cyclic trend denotes change in disease occurrence over a short period of time usually two to three years. This is a characteristic of infectious diseases which confers long lasting immunity.

• Secular trend denote changes in disease incidence occurring over periods of many years.

Descriptive studies-space and time

Space-time clustering: Disease can be described by both place and time variables Many infectious diseases, occurring in one geographical area, tend to occur at around the same time. Such a phenomenon can occur by chance in non-infectious diseases. Cluster analysis techniques are used to determine whether the degree of space-time clustering observed for any disease is pertinent to the infectious disease etiology. Time-space clusters are therefore a useful clue to the etiology of certain infectious diseases.

ADVANTAGES OF DESCRIPTIVE STUDIES

• o They are quick to be done, (most often you use routine data already available)

• o They are cheap to conduct.• o They lead to formulation of hypotheses

to be tested with analytical studies.

DISADVANTAGES OF DESCRIPTIVE STUDIES

Unable to link exposure with disease in particular individuals, eg. the increase of the Pap smear rate and decrease of the mortality of the cancer of cervix.

Lack of ability to control for the effects of potential confounding factors, e.g. there is a very strong correlation between per capita television sets and coronary heart disease (CHD) in various countries.

DISADVANTAGES OF DESCRIPTIVE STUDIES 2

Correlation data represent average exposure levels rather than actual individual values, e.g. CHD rate showed a strong negative correlation with national per capita alcohol alcohol consumption; countries with high alcohol consumption showed a low risk of sudden death from coronary heart disease.

Disadvantages of descriptive studies 3

Case reports cannot test for the presence of valid statistical association.

They are usually based on the experience of one individual.

ANALYTICAL CROSSECTIONAL STUDIES

ANALYTICAL CROSSECTIONAL STUDIES MEASURE THE PREVALENCE OF DISEASES OR HEALTH PROBLEMS IN THE COMMUNITY AS WELL AS TESTING ONE OR SEVERAL HYPOTHESES FOR CAUSATION OF DISEASE.

TO TEST A HYPOTHESIS THE SAMPLE HAS TO PROVIDE BOTH STUDY AND CONTROL GROUP.

INDICATIONS OF ANALYTICAL CROSSECTIONAL STUDIES

• STUDIES THE PREVALENCE OF A DISEASE AS WELL AS SEARCH FOR WHAT MIGHT BE THE CAUSATIVE FACTOR OR CAUSATIVE FACTORS.

• USEFUL FOR TESTING A HYPOTHESIS ABOUT EXPOSURES, WHICH ARE PERMANENT OR COMMON FEATURE FOR INDIVIDUALS, E.G , EDUCATION, SOCIOECONOMIC STATUS.

• IN A SUDDEN DISEASE EPIDEMIC MEASUREMENTS OF POTENTIAL CAUSATIVE FACTORS IS OFTEN THE MOST CONVINIENT FIRST STEP IN THE INVESTIGATION OF THE CAUSE OF THE EPIDEMIC

• THEY ARE CHEAP AND OF SHORT DURATION

Advantages of analytical Crossectional Studies

• TESTING MULTIPLE EFFECTS OF CAUSE AS WELL AS TESTING CAUSES FOR AN EFFECT - GOOD

• SELECTION BIAS - LOW

• LOSS TO FOLLOW UP - VERY LOW

• TIME REQUIRED - SHORT

• COST - LOW

Disadvantages of analytical crossectional studies

• Investigation of rare disease (or problem) - Not possible

• Investigation of rare cause (exposure) - Not possible

• Measure of time relationship (increase or decrease over time) - Not possible

• Measurement of incidence - Not possible

• Recall bias - Very high

Steps in carrying out a crossectional study

1. Choosing a problem for study

2. Review of past literature

3. Statement of the research problem

4. Rationale or justification of the research problem

5. Specification of the aim of study. Aim should be relevant to disease control or management.

Steps in carrying out a crossectional study 2

6. Definition of the study population.

7. Sample size determination. Sample size should be adequate for valid estimates.

8. Sampling,

9. Recruitment and management of sample in order to achieve a high response rate which is important for valid estimates.

Steps in carrying out a crossectional study 3

10. Examination, interviews and record retrieval methods.

11.Data handling and management.

12. Analysis, interpretation of the results

13. report writing.

14. Publication

BRAIN STORMING

• WHAT IS A RARE ?• WHAT IS COMMON DISEASE?

• WHAT IS A RARE EXPOSURE?

• WHAT IS A SELECTION BIAS?• WHAT IS A RECALL BIAS ?

• IF WE STUDY PREVALENCE OF A DISEASE IN A CROSSECTIONAL STUDY WHAT DO WE STUDY IN A COHORT STUDY?

What type of study is the following?

• A total of 250 adults were interviewed in Moshi Municipality. Over half (62.8%) of the respondents admit to take alcohol, most of them (about 60%) started taking alcohol at the age below 20 years of age.

• The highest proportion of alcohol users was between 24-29 age group. A few drinkers (28.7%) experience symptoms of dependence and about 45.2% have symptoms of tolerance.

What type of study produced the following data?

• In the Muhimbili (Dar es Salaam) Cancer Registry, between 1974 and 1995, there were registered 13,008 cancers in males and 19,557 cancers in females, or overall 2.7% of the 453,023 males, and 2.6 of 766,539 females patients, treated in that hospital . The male : female sex ratio was 1:1.6.


• The Cancer Registry based in Muhimbili also show top 4 cancers in males to be oesophagus 9.7%, prostate 8.4%, liver 6.9% and larynx 6.1. , while the top female cancers are cervix 37.3%, breast 11.2%, lymphatic leukemia 3.6% and stomach 3.1%.

• In the Cancer Registry, based at KCMC hospital the top 4 male cancers were liver- 9.20%, prostate- 6.5%, stomach - 5-6% and lymphosarcoma - 3.5%, while in females the top 4 cancers were cervix - 9.4%, breast - 4.7%, stomach - 4.6% and connective tissue -2.5%.


• The HIV/AIDS epidemic seem to be a major factor for increase of cancers in Tanzania and other developing countries. Kaposi Sarcoma, for example, has increased from 3.6 per 100,000 males to 36.2 per 100,000 males, per year and in females from 0.5 to 11.2 per 100,000 females per year, in Tanzania (Kitinya et al 1999, age standardized rate). In Uganda corresponding increase in Kaposi Sarcoma is 2.6 to 30 per 100,000 males per year and 0.0 to 11.0 per 100,000 females per year.


• Shistosomiasis in absence of Human Papilloma Virus (HPV) is not a cause of cancer of the cervix uteri. Over 10 years period (1980 - 1990) 4520 cases of cervix cancer were reviewed. Only 76 (17%) had schistosomiasis, and 26 of them showed HPV positive.

What type of study is to be done following the following objectives?General Objectives were:

To assess the use of SP and other antimalarial drugs for home treatment of malaria in Same district, Tanzania

Specific Objectives:• To determine the availability of SP and other antimalarial

drugs in health facilities, shops and households.• To determine the knowledge and practice of SP and

other antimalarial drugs by mothers and other care givers use for home treatment of malaria.

• To assess the knowledge and skills of health workers in advising people about home treatment of malaria.

What type of study is this?

General Objectives were: To evaluate the impact of SP packaging for malaria

treatment in under five children in Same district. Specific Objectives:• To educate health workers and CORPs on use of SP

for treatment of malaria.• To educate caregivers, drug sellers and

shopkeepers in proper use of SP, using packages for different age groups.

• To evaluate the impact of these measures in the overall management of malaria and its outcome in under 5 children.

Formulation of a hypothesis

Method of difference; if the disease frequency is significantly different between two sets of circumstances the disease might have a causal association with a particular factor that differs between the two

Method of agreement;

If single factor is common in a number of circumstances in which the disease occurs causal association can be suspected.

Method of concomitant variation

If a factor varies in proportion to the frequency of disease causal

association can also be suspected.

Crossectional studies 2

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