Misplaced Pages

#876123

29-443: Based on the type of muscles affected, spinal muscular atrophies can be divided into: When taking into account prevalence , spinal muscular atrophies are traditionally divided into: A more detailed classification is based on the gene associated with the condition (where identified) and is presented in table below. In all forms of SMA (with an exception of X-linked spinal muscular atrophy type 1 ), only motor neurons , located at

58-576: A   d i s e a s e t h e   t o t a l   t i m e   a t   r i s k   f o r   a l l   p e o p l e   t o   g e t   t h e   d i s e a s e {\displaystyle Incidence\ rate={\frac {the\ number\ of\ subjects\ developing\ a\ disease}{the\ total\ time\ at\ risk\ for\ all\ people\ to\ get\ the\ disease}}} One of

87-405: A s e   o v e r   a   c e r t a i n   p e r i o d t h e   t o t a l   n u m b e r   o f   s u b j e c t s   f o l l o w e d   o v e r   t h

116-410: A t   p e r i o d {\displaystyle Incidence={\frac {number\ of\ subjects\ developing\ the\ disease\ over\ a\ certain\ period}{the\ total\ number\ of\ subjects\ followed\ over\ that\ period}}} For example, if a population contains 1,000 persons and 28 develop a condition from the time the disease first occurred until two years later, the cumulative incidence

145-483: A 10-year period: If you were to measure prevalence you would simply take the total number of cases (25 + 20 + 30 = 75) and divide by your sample population (225). So prevalence would be 75/225 = 0.33 or 33% (by the end of the study). This tells you how widespread HIV is in your sample population, but little about the actual risk of developing HIV for any person over a coming year. To measure incidence rate you must take into account how many years each person contributed to

174-500: A low incidence yet will continue to have a high prevalence (because it takes a long time to cure, so the fraction of individuals that are affected remains high). In contrast, a disease that has a short duration may have a low prevalence and a high incidence. When the incidence is approximately constant for the duration of the disease, prevalence is approximately the product of disease incidence and average disease duration, so prevalence = incidence × duration . The importance of this equation

203-430: A relevant, non-negligible number of subjects who are incorrectly classified as having the condition or any other condition which is the object of a survey study: these subjects are the so-called false positives; such reasoning applies to the 'false positive' but not the 'false negative' problem where we have an error applied over a relatively very small number of individuals to begin with (that is, those who are affected by

232-405: A season, or a year. Point prevalence can be described by the formula: Prevalence = Number of existing cases on a specific date ÷ Number of people in the population on this date It can be said that a very small error applied over a very large number of individuals (that is, those who are not affected by the condition in the general population during their lifetime; for example, over 95%) produces

261-421: Is "the proportion of a population that might become afflicted with a given disease at any point in their lifetime." Period prevalence is the proportion of the population with a given disease or condition over a specific period of time. It could describe how many people in a population had a cold over the cold season in 2006, for example. It is expressed as a percentage of the population and can be described by

290-475: Is 28 cases per 1,000 persons, i.e. 2.8%. The incidence rate can be calculated by dividing the number of subjects developing a disease by the total time at risk from all patients: I n c i d e n c e   r a t e = t h e   n u m b e r   o f   s u b j e c t s   d e v e l o p i n g  

319-426: Is a measure of the proportion of people in a population who have a disease or condition at a particular time, such as a particular date. It is like a snapshot of the disease in time. It can be used for statistics on the occurrence of chronic diseases . This is in contrast to period prevalence which is a measure of the proportion of people in a population who have a disease or condition over a specific period of time, say

SECTION 10

#1732859413877

348-405: Is analogous to a long exposure (seconds, rather than an instant) photograph: the number of events recorded in the photo whilst the camera shutter was open. In a movie each frame records an instant (point prevalence); by looking from frame to frame one notices new events (incident events) and can relate the number of such events to a period (number of frames); see incidence rate . Point prevalence

377-473: Is distinct from incidence . Prevalence is a measurement of all individuals affected by the disease at a particular time, whereas incidence is a measurement of the number of new individuals who contract a disease during a particular period of time. Prevalence is a useful parameter when talking about long-lasting diseases, such as HIV , but incidence is more useful when talking about diseases of short duration, such as chickenpox . Lifetime prevalence (LTP)

406-420: Is in the relation between prevalence and incidence; for example, when the incidence increases, then the prevalence must also increase. Note that this relation does not hold for age-specific prevalence and incidence, where the relation becomes more complicated. Consider the following example. Say you are looking at a sample population of 225 people, and want to determine the incidence rate of developing HIV over

435-418: Is most often used in questionnaire studies. Prevalence is the number of disease cases present in a particular population at a given time, whereas incidence is the number of new cases that develop during a specified time period. Prevalence answers "How many people have this disease right now?" or "How many people have had this disease during this time period?". Incidence answers "How many people acquired

464-414: Is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number of people found to have the condition with the total number of people studied and is usually expressed as a fraction, a percentage, or the number of cases per 10,000 or 100,000 people. Prevalence

493-462: Is the proportion of individuals in a population that at some point in their life (up to the time of assessment) have experienced a "case", e.g., a disease; a traumatic event; or a behavior, such as committing a crime. Often, a 12-month prevalence (or some other type of "period prevalence") is provided in conjunction with lifetime prevalence. Point prevalence is the prevalence of disorder at a specific point in time (a month or less). Lifetime morbid risk

522-422: Is usually more useful than prevalence in understanding the disease etiology: for example, if the incidence rate of a disease in a population increases, then there is a risk factor that promotes the incidence. For example, consider a disease that takes a long time to cure and was widespread in 2002 but dissipated in 2003. This disease will have both high incidence and high prevalence in 2002, but in 2003 it will have

551-436: The anterior horn of spinal cord , are affected; sensory neurons , which are located at the posterior horn of spinal cord , are not affected. By contrast, hereditary disorders that cause both weakness due to motor denervation along with sensory impairment due to sensory denervation are known as hereditary motor and sensory neuropathies (HMSN). Creutzfeldt–Jakob disease Prevalence In epidemiology , prevalence

580-431: The receiver operating characteristic curve), a condition with a relatively low prevalence or base-rate is bound to yield high false positive rates, which exceed false negative rates; in such a circumstance a limited positive predictive value , PPV, yields high false positive rates even in presence of a specificity which is very close to 100%. Incidence (epidemiology) In epidemiology , incidence reflects

609-487: The condition in the general population; for example, less than 5%). Hence, a very high percentage of subjects who seem to have a history of a disorder at interview are false positives for such a medical condition and apparently never developed a fully clinical syndrome . A different but related problem in evaluating the public health significance of psychiatric conditions has been highlighted by Robert Spitzer of Columbia University : fulfillment of diagnostic criteria and

SECTION 20

#1732859413877

638-403: The disease [during a specified time period]?". However, mathematically, prevalence is proportional to the product of the incidence and the average duration of the disease. In particular, when the prevalence is low (<10%), the relationship can be expressed as: Caution must be practiced as this relationship is only applicable when the following two conditions are met: 1) prevalence is low and 2)

667-410: The disease, whereas prevalence indicates how widespread the disease is. Prevalence is the proportion of the total number of cases to the total population and is more a measure of the burden of the disease on society with no regard to time at risk or when subjects may have been exposed to a possible risk factor. Prevalence can also be measured with respect to a specific subgroup of a population. Incidence

696-421: The duration is constant (or an average can be taken). A general formulation requires differential equations . In science, prevalence describes a proportion (typically expressed as a percentage ). For example, the prevalence of obesity among American adults in 2001 was estimated by the U. S. Centers for Disease Control (CDC) at approximately 20.9%. Prevalence is a term that means being widespread and it

725-433: The following formula: Period prevalence (proportion) = Number of cases that existed in a given period ÷ Number of people in the population during this period The relationship between incidence (rate), point prevalence (ratio) and period prevalence (ratio) is easily explained via an analogy with photography. Point prevalence is akin to a flashlit photograph: what is happening at this instant frozen in time. Period prevalence

754-400: The important advantages of incidence rate is that it doesn't require all subjects to be present for the whole study because it's only interested in the time at risk. Incidence should not be confused with prevalence , which is the proportion of cases in the population at a given time rather than rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting

783-594: The number of new cases of a given medical condition in a population within a specified period of time. Incidence proportion ( IP ), also known as cumulative incidence , is defined as the probability that a particular event, such as occurrence of a particular disease, has occurred in a specified period: I n c i d e n c e = n u m b e r   o f   s u b j e c t s   d e v e l o p i n g   t h e   d i s e

812-460: The resulting diagnosis do not necessarily imply need for treatment. A well-known statistical problem arises when ascertaining rates for disorders and conditions with a relatively low population prevalence or base rate . Even assuming that lay interview diagnoses are highly accurate in terms of sensitivity and specificity and their corresponding area under the ROC curve (that is, AUC , or area under

841-603: The study, and when they developed HIV because when a subject develops HIV he stops being at risk. When it is not known exactly when a person develops the disease in question, epidemiologists frequently use the actuarial method, and assume it was developed at a half-way point between follow-ups. In this calculation: That is a total of (1500 + 275) = 1775 person-years of life. Now take the 50 new cases of HIV, and divide by 1775 to get 0.028, or 28 cases of HIV per 1000 population, per year. In other words, if you were to follow 1000 people for one year, you would see 28 new cases of HIV. This

#876123