approaches and basic measurement in epidemiology

Approaches and Basic measurement in Epidemiology

• Epidemiological approach to problem of health and disease is based on 2 major foundations

• asking questions• making comparison

asking questions

• Health related events-what is the event/problem?-what is its magnitude?-where did it happen?-when did it happen?-who are affected?-why did it happen?

asking question• Related to health action-what can be done to reduce the problem and its consequences?-How can it be prevented in future?-what action should be taken by health sectors/other

sectors/community?-what resources are required? what are the activities to be

organized?-what difficulties may arise and how might they be overcome?

Answer to the above questions may provide clues to disease etiology and help to guide planning and evaluation

Making comparison

• making comparison and drawing inferences is another approach

-comparison may be between two or more groups

-finding out any crucial differences in the host and environmental factors between those effected and not effected

-clues to aetiology from such comparisonFor comparison we need measurement

“If you can measure that of which you speak, and can express it by a number, you know something of your subject, but if you can not measure it, your knowledge is meager and unsatisfactory.” —Lord Kelvin (1824–1907)

Scales of measurement orLevel of measurement

Measurement Involves measuring an attribute or property of a

person, object or event according to a particular set of rules

The measurement process results in a number There must be a rigorous set of rules for assigning

numbers to the objects being measured Through the measurement process, we transform

data into information

Quantification

• An important first-step in measurement is determining whether a variable is discrete or continuous.

• Why? This determines how we quantify or measure the variable.

• Variable: A feature for which people differ. – Height: some people are shorter than others– Age: some people are older than others

Scales of measurement To properly assign numbers one must understand the

scales of measurement The scale determines the amount of information

contained in the data. Understanding the scales of measurement results in

the appropriate selection of graphic techniques and procedures for data analysis

Scales of measurement

• Nominal• Ordinal• Interval• Ratio

Nominal scale• Labels or names used to identify an attribute of the

element. • A nonnumeric label or a numeric code may be used.• Examples:– country of origin– biological sex (male or female)– animal or non-animal– married vs single

Nominal scale

• Sometimes numbers are used to designate category membership

• Example: Country of Origin1 = United States 3 = Canada2 = Mexico 4 = Nepal

• However, in this case, it is important to keep in mind that the numbers do not have intrinsic meaning

Ordinal

– The data have the properties of nominal data and the order or rank of the data is meaningful.

– A nonnumeric label or a numeric code may be used.

– Categories are ordered, but not numeric; intervals between categories are not equal

Ordinal

• Designates an ordering• Does not assume that the intervals between numbers are

equal• Example 1:

finishing place in a race (first place, second place)

1 hour 2 hours 3 hours 4 hours 5 hours 6 hours 7 hours 8 hours

1st place 2nd place 3rd place 4th place

Ordinal

Example 2: Ordinal scales — have some order

Class Rank Patient Condition1. Freshman 1. Mild2. Sophomore 2. Moderate3. Junior 3. Severe4. Senior 4. Critical

Interval

– The data have the properties of ordinal data and the interval between observations is expressed in terms of a fixed unit of measure but lack a real zero point (numerical)

– Its most important characteristic is that the intervals between successive values are equal

– The zero point on this scale is arbitrary.• Calendar time• Temperature

Interval

Example: Common IQ tests The difference between someone with a score is 120 and

someone with a score of 100 is the same as the difference between people with scores of 80 and 60 (i.e., 20 points)

Ratio scale

• The data have all the properties of interval data and the ratio of two values is meaningful.– Designates an equal-interval ordering with a true

zero point (i.e., the zero implies an absence of the thing being measured)

– Variables such as distance, height, weight, use the ratio scale.

Ratio scale

Example: height, weight, Ram is 3.3 ft tall, Shyam is 6.6 feet tall,

Shyam is twice as tall as Ram.

Tools of measurement

• Basic tools of measurement in epidemiology are

-Ratios-Proportions and-Rates Absolute number conveys no meaning to an

epidemiologist who is interested in comparing the frequency of events. So, these are necessary.

RateRate: Rate is an instantaneous change in one quantity per unit

change in another quantity, where the latter is usually a time

xKba

aRate

where, a = number of times an event has occurred in a specific interval of timea+b = number of persons exposed to risk of the event during thesame interval (a is a portion of a+b)k = some round number (100 or 1,000 etc.) or base, depending uponthe relative magnitude of a and a+b

Characteristics of rate

a. It has unit of measurement (with dimension) i.e. time.

b. It has no finite bound. Theoretically, a rate can approach infinity.

Ratio: A ratio is the expression of the relationship between a numerator and denominator, which may involve either an interval in time or may be instantaneous change in time

y

xRatio

x= number of events or items counted and not necessarilya portion of yy= number of events or items counted

There are two kinds of ratio:

a. One that has dimension (unit of measurement) e.g. number of hospital beds per 100, 000 persons in population, number of infant deaths in population during one year per 1000 live births

b. One that is dimensionless (has no unit of measurement) e.g. by dividing one proportion or rate to another

Proportion: A proportion is an expression in which the numerator is always included in the denominator and the base is equal to 100.

Characteristics of proportion

a. It is dimensionless (no unit of measurement). Since numerator and denominator have the same dimension, any dimensional contents are cancelled out

b. The value ranges between 0 and 1 (0.0< p <1.0)

• Ratio express a relation in size between 2 random quantities

• Numerator is not a component of denominator

• result of dividing one quantity by another• expressed in the form of x y or x/y• example sex ratio, doctor population ratio,

• Proportion• indicates the relation in magnitude between a

part and the whole• the numerator is always included in the

denominator• usually expressed as a percentage• proportion of EP cases out of total TB cases

EP/Total TB*100 N%

Rate• a rate measures the occurence of some particular event in a

population during a given time period• It is a statement of the risk of a developing a condition• A rate comprises 4 elements-numerator-denominator-time specification specially a calendar year and -multiplier to express per 100,1000,or other round figureexample : death rate= no. of death in one year X1000/mid yr popFor ensuring national and international comparibility , it is very

necessary to have a uniform and standardized system of recording and classifying disease and death.

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