the attached pages from Rothman and Grennland Modern epidemiology 2nd ed 1998 might be helpful

Hello James Leigh,

Thank you so much for that, it is so helpful.

You can model Your observation of a residential euqilibrium between prevalence, incidence and mortality by a Markoff-Chain-Model. You find a simulation at slide 19 of my lecture "Systemic Lupus erytematosus Lianyungang 07/20118 Ni Hao" with data of prevalence and incidence and comparison of the findings in the Feng & Pan & Wu 2016 study.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0168619

If the mortality of the observed disease is high, the prevalence drops. It drops down to the equality of incidence and prevalence.

Hello Martin Wedig,

Thank you so much for that, in fact it is much useful.

Actually it is not a relationship, rather it makes a difference. These are studies that differ because they are transversal (prevalence) and longitudinal (incidence). When making the prevalence, the characteristics of variables are estimated at a specific moment. When making an incidence, the increase or decrease (variable behavior) is estimated at two different times (repetition of sampling) in the same study group.

I have attached a part of veterinary epidemiology book (Thrusfield) which explain the relationship.

Hello Mohammad hasan Rabiee & José J. Him Fábrega

I really appreciate for providing me with this stuff, it is really helpful.

Thanks.

All of the above answers are precise. In addition to these, Prevalence is also affected by Mortality rate. If you look at acute illness with high CFR;, point prevalence can be very low.

This thread gathers many aspects about prevalence and incidence. Although they show two orthogonal dimensions (J.J. Him F.), they are connected (M.H. Rabiee) by Mortality (T. Lakiang). This results in the chance of calculating invisable compartments by Markow-Chain-Analysis (M.P. Wedig). The experience view is given by linked literature (J. Leigh). So the initial question for relation (S.K. Gudi) resulted in an enlarged functional view.

Incidence and prevalence are point processes, and some additional assumptions may provide a simpler relationship between them. The Rothman and Greenland book is always a place to start.

My views are that, Incidence should not be confused with prevalence, which is the proportion of cases in a specified population at a given time rather than rate of occurrence of new cases. Therefore, incidence conveys information about the risk of contracting the disease in a population, whereas prevalence indicates how widespread the disease is in that population.

Incidence are the new diagnosed cases and the prevalence is the cumulative cases of one disease compared with the total population studied

In addition to what my colleagues had said! Incidence measures instances of a factor such as disease, injury, health status during a given period (day, week, month, year, decade) in a specified population it could be either by age group, community or country while while Prevalence gives figure for a factor (disease, health status) at a single point in time or time period. Incidence also conveys information about the risks of contracting the disease and indicates how widespread the disease is.

Yes, Mr. Ibaku is correct. That is the definition of incidence and prevalence.

The dynamic relation of incidence (I) and prevalence (P) connected by the mean duration (D) of a disease is shown in the formula

P/(1-P) = I * D

1-P is the propability of being not sick.

If prevalence is very small the formula is

P = I * D.

The CDC cours of epidemiology pencils an introduction in applied epidemiology and biostatistitics:

https://www.cdc.gov/ophss/csels/dsepd/ss1978/

I think Mr. Ibaku have spelled out the differences between "Incidence" and "Prevalence".

Dear Sai

I will give details on incidence and prevalence which I thought be useful for you. This is a summary lecture I normally deliver to medical studnts.

Rates related to morbidity: These are used to measure the frequency

of disease in the population at one point in time or during a period of

time. They are very important because they reflect the health status of

people and the expected burden of disease on the health care system. Two morbidity rates are in common use:

Incidence rate: Incidence of a disease is the number of new cases (number of persons who get diseased) or episodes of disease which occur during a specified period of time in a specific population or place. The incidence rate (IR) is the number of new cases or episodes (spells) of disease per unit of size of population.

Number of new cases of a disease in a year in a given

population

IR = ------------------------------------------------------------------ X 1000

Total population at risk in the same year

Or

Number of new spells of disease in a year in a given

population

IR =----------------------------------------------------------------------- X 1000

Total population at risk in the same year

Which of these two rates do you expect to have higher value if both are calculated for the same population during a given year?

Incidence rate is more useful in the following situations:

To study disease of short duration.

To study the aetiology of disease.

To evaluate preventive measures.

To determine the risk of acquiring of disease.

To assess transmission of infectious agent.

Prevalence rate: Prevalence refers to the total number of cases of a disease or conditions existing in a given population at a point in time (point prevalence) or during a period of time (period prevalence). According to time specification, prevalence rate (number of existing cases per unit of size of population) is of two types:

Point prevalence rate (PnPR): it is the commonly used prevalence rate and measures the probability of disease existence at a point in time in a given population

Number of existing cases (new & old) in a given population

at a point in time

PnPR= ------------------------------------------------------------------------ X 1000

Total population in the same place and the same point in

time

Period prevalence rate (PrPP): Less commonly used. The numerator includes all cases (new & old) existing in population during a given period of time.

Number of existing cases during a period of time

PrPR=------------------------------------------------------------------------- X 1000

Total population at risk in the same place and time period

Notes:

-The term “point in time” may be as long as it takes to get information on existing cases in cross-sectional studies.

- Period prevalence during a year is equal to combination of all cases existing at the beginning of the year plus all new cases which occur during the year regardless of their fate (death, recovery or infirmity).

Prevalence rates are useful for:

1. Diseases of long duration.

2. Administrative purposes (e.g., planning of health services).

Relationship of incidence and prevalence

Prevalence of disease may vary from place to place or from time to another because of variation in incidence and/ or duration of the disease. In an epidemiologically stable situation, i.e, with constant incidence and duration of disease, the following relationship may be stated:

Prevalence = Incidence X Duration

Duration of a disease is a function of its fatality and its tendency to recover. The higher the case fatality of a disease, the shorter the average duration of it. Similarly, the quicker the recovery of the disease, the shorter the duration is.

Actually point prevalence of any disease is a function of its incidence rate and the rate at which cases die or completely recover.

Hay everyone,

I really appreciate each one you for showing concern towards my question and providing with your opinions, especially they all are much useful though.

Thanks again.

I fully agree with the criteria applied by Dr Ibaku