LESSON 6

LESSON 6. THE EMPIRICAL STAGE OF THE INVESTIGATION:

The design, material and methods

1. MATERIAL AND METHODS.

Study population: Selection of individuals in search of internal and external validity (avoid selection biases...).

Sampling: When it is not possible to incorporate the entire study population. Shall be deemed:

• Sample size: To make inference with a given error

• Representation: simple, systematic, stratified, random sampling by clusters...

2.       FREQUENCY MEASURES IN DESCRIPTIVE STUDIES.

What do we want to measure? :

The situation at a given point in time: PREVALENCE MEASURE

What is happening over a period of time: INCIDENCE.

Characteristics of the incidence:

• It is dimensional

• Adopts values ​​between 0 and infinity (it is a rate)

Accumulated incidence: calculates the risk of a phenomenon occurring. It measures the average risk of suffering from the disease.

- Measures the probability of having the event.

- It has no units. It is a proportion (expressed as %)

- Values ​​between 0 and 1 [0-100]

- Does not imply the period of time: should always be expressed.

- There can be no losses in follow-up

- All subjects are followed throughout the period

Incidence rate or incidence density: Refers to the speed with which the new cases appear with respect to the size of the population. Often not all at-risk individuals (denominator) are followed for the same time. If the different observation times ("times at risk") of the different individuals are available, the incidence density or incidence rate can be calculated.

3. FOLLOW-UP AND EXPERIMENTAL STUDIES.

4. CASE STUDIES AND CONTROLS. ESTIMATION OF THE MAGNITUDE OF ASSOCIATION.

Two groups are compared: one that presents the dependent variable and another control group that does not present it. In these studies there is neither incidence nor prevalence because it is investigated in its past.