To complete your formula, you need to multiply this proportion by a base 10 (10^n). The "n" value depends on the size of the population and the frequency with which the event occurs. For very frequent events, we usually use smaller "n" values (100; 1000 ...); for infrequent events we used larger "n" values (100,000, 1,000,000 for example).

Another factor that influences the choice of the best indicator (incidence or prevalence) for the duration of the disease "cycle" - for short cycle diseases (acute respiratory infections, acute diarrheal diseases, etc.) we usually work with Incidence Rates; (such as arterial hypertension, diabetes mellitus, etc.), we use the Prevalence Rates

Thank you so much Clayton Coelho

Here are simplified formulas for calculation of incidence and prevalence rates:

RATE OF INCIDENCE for certain disease means occurring of new cases of disease in defined population in certain period of time.

Number of new cases diseased from certain disease

INC = ------------------------------------------------------------------------------------------------ *K

Number of inhabitants (under risk)

RATE OF PREVALENCE shows the total number from certain disease (new and old cases) in certain population for defined period of time (period of prevalence).

Number of diseased from certain disease (all cases)

PREV = ------------------------------------------------------------------------------------------------- *K

Number of inhabitants (under risk)

"K" value is equivalent to "n" value as explained by Clayton Coelho. Such value should be constant for comparison of rates in certain periods of time or between different territories or populations.

Rate of prevalence we might calculate for chronic diseases (diabetes, cancer, hypertension, renal failure etc.) and for some determinants/consequences of disease (diabetic gangrena, blindness etc.). We might also calculate a rate of prevalence of impairments, disability and invalidity in the population.

Dear Sir, thank you so much for your detail answer.

And, some additional explanation about the meaning of incidence and prevalence.

High incidence of certain disease means that prevention is weak and measures and activities to prevent and control the disease are insufficient.

High prevalence of certain disease means that treatment is weak. It is also an indicator for the severity of the health problem or certain disease.

Incidence refers to new cases (subjects with a disease, condition..etc) or new episodes of a disease or condition. Incidence rate is the number of new cases or new episodes of disease per 1000 population at risk. Thus incidence rate (IR) is calculated according to the formula given below which is not very much different from previous recommendations

No. of new cases/episodes

------------------------------------------- X1000

Population at risk

The multiplication factor is normally 1000 but may be 10000 , 100000 or even 1000000

Prevalence refers to the number of existing cases regardless of date of onset. Prevalence rate is the number of existing cases at a point or a period of time per 1000 population.

Thanks for your kind explanation.

I agree with my colleagues. The denominator of the incidence is the'population who are at risk to develop a disease or health condition' and who are free from the disease at the start of the period. e.g if you calculate the incidence of measles during 2017, the denominator should be people who are susceptible to measles, and who are free from measles on Jan 1, 2017. The numerator is the number of new cases of measles developed during 2017. Then you have to multiply by a constant e.g 100000. Note that the incidence is usually measured for communicable diseases, and the prevalence is usually measured for chronic non-communicable diseases.

Thanks for such a nice explanation.

http://www.differencebetween.net/science/health/difference-between-prevalence-and-incidence/

https://www.researchgate.net/post/What_is_the_main_difference_between_Incidence_and_Prevalence

You might find both the references useful

I agree with all authors especially with Doncho Donev.

The last sentence in the answer of Namir G Al-Tawil - "Note that the incidence is usually measured for communicable diseases, and the prevalence is usually measured for chronic non-communicable diseases", needs some additional explanation.

In fact, the rate of incidence means the rate of morbidity for all acute or short term diseases or events related to health, first of all communicable diseases with short duration of a few days or a few weeks, rarely a few months, but also some other acute events, i.e. injuries, heart attacks, strokes etc.

For non-communicable diseases, and for chronic communicable diseases i.e tuberculosis, malaria etc., in order to provide profound insight we have to calculate two indicators - incidence and prevalence - as explained above.

And, the constant might be 1.000 or 10.000 for more frequent diseases or 100.000 or even 1.000.000 for less frequent diseases. For example, the number of new and total cases of renal failure or patients on dialysis program are presented per 1.000.000 inhabitants.

Very edifying thank you !

Hi. First you must bear in mind that it is a longitudinal study. It means that a sampling should be done at one time and then another from the same population. The increase or decrease of cases determines the incidence.

Couldn’t agree more with answers above, hope that answered you questions

The incidence rate is calculate as follows: Number of New cases in the population multiply by 100% and divide by the total number of the study population at that moment in time.

Good question, have more progress dear colleague