Mark and recapture
Encyclopedia
Mark and recapture is a method commonly used in ecology
to estimate population
size. This method is most valuable when a researcher fails to detect all individuals present within a population of interest every time that researcher visits the study area. Other names for this method, or closely related methods, include capture-recapture, capture-mark-recapture, mark-recapture, sight-resight, mark-release-recapture, multiple systems estimation and band recovery.
Another major application for these methods is in epidemiology
, where they are used to estimate the completeness of ascertainment of disease registers. Typical applications include estimating the number of people needing particular services (i.e. services for children with learning disabilities, services for medically frail elderly living in the community), or with particular conditions(i.e. illegal drug addicts, people infected with HIV
, etc.).
Sufficient time is allowed to pass for the marked individuals to redistribute themselves among the unmarked population.
Next, the researcher returns and captures another sample
of individuals. Some of the individuals in this second sample will have been marked during the initial visit and are now known as recaptures. Other animals captured during the second visit will not have been captured during the first visit to the study area. These unmarked animals usually are given a tag or band during the second visit and then are released.
Population size can be estimated from as few as two visits to the study area. Commonly, more than two visits are made, particularly if estimates of survival or movement are desired. Regardless of the total number of visits, the researcher simply records the date of each capture of each individual. The "capture histories" generated are analyzed mathematically to estimate population size, survival, or movement.
In the epidemiological setting, different sources of patients take the place of the repeated field visits in ecology
. To take a concrete example, establishing a register of children with Type 1 diabetes children were identified from hospital admission records, from general practitioners (family doctors), and from the records of the local Diabetes Association. None of these sources had a complete list, but by putting them together it was possible to do two things, first to see how many children were identified in total, and secondly to estimate how many more children with Type 1 diabetes were living in the vital community
) can be used to estimate population size if only two visits are made to the study area. This method assumes that the study population is "closed." In other words, the two visits to the study area are close enough in time so that no individuals die, are born, move into the study area (immigrate) or move out of the study area (emigrate) between visits. The model also assumes that no marks fall off animals between visits to the field site by the researcher, and that the researcher correctly records all marks.
Given those conditions, estimated population size is:
where
This implies that, in the second sample, the proportion of marked individuals that are caught (
) should equal the proportion of the total population that is caught (
). For example, if half of the marked individuals were recaptured, it would be assumed that half of the total population was included in the second sample.
In symbols,
A rearrangement of this gives
the formula used for the Lincoln–Petersen method (Krebs, 1998).
In this example, the Lincoln–Petersen method estimates that there are 30 turtles in the lake.
A less biased estimator
of population size can be obtained with a modified version of the first formula above:
where, as before,
An approximately unbiased variance of N, or var(N), can be estimated as:
model. Sophisticated mark-recapture models can be fit using Recapturehttp://cran.r-project.org/web/packages/Rcapture/index.html, a package of the Open Source R programming language, or specialized programs such as MARK http://www.warnercnr.colostate.edu/~gwhite/mark/mark.htm or M-SURGE http://www.cefe.cnrs.fr/biom/Logiciels.htm. The other related methods which are often used, include Jolly-Seber method (used in open populations and for multiple census estimates) and Schnabel estimates (described above as an expansion to the Lincoln-Petersen method for closed populations). These are described in detail by Sutherland.
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
to estimate population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
size. This method is most valuable when a researcher fails to detect all individuals present within a population of interest every time that researcher visits the study area. Other names for this method, or closely related methods, include capture-recapture, capture-mark-recapture, mark-recapture, sight-resight, mark-release-recapture, multiple systems estimation and band recovery.
Another major application for these methods is in epidemiology
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
, where they are used to estimate the completeness of ascertainment of disease registers. Typical applications include estimating the number of people needing particular services (i.e. services for children with learning disabilities, services for medically frail elderly living in the community), or with particular conditions(i.e. illegal drug addicts, people infected with HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
, etc.).
Field work related to mark-recapture
Typically a researcher visits a study area and uses traps to capture a group of individuals alive. Each of these individuals is marked with a unique identifier (e.g., a numbered tag or band), and then is released unharmed back into the environment. A mark recapture method was first used for ecological study in 1896 by Petersen to estimate plaice, Platichthys platessa, populations.Sufficient time is allowed to pass for the marked individuals to redistribute themselves among the unmarked population.
Next, the researcher returns and captures another sample
Sample (statistics)
In statistics, a sample is a subset of a population. Typically, the population is very large, making a census or a complete enumeration of all the values in the population impractical or impossible. The sample represents a subset of manageable size...
of individuals. Some of the individuals in this second sample will have been marked during the initial visit and are now known as recaptures. Other animals captured during the second visit will not have been captured during the first visit to the study area. These unmarked animals usually are given a tag or band during the second visit and then are released.
Population size can be estimated from as few as two visits to the study area. Commonly, more than two visits are made, particularly if estimates of survival or movement are desired. Regardless of the total number of visits, the researcher simply records the date of each capture of each individual. The "capture histories" generated are analyzed mathematically to estimate population size, survival, or movement.
In the epidemiological setting, different sources of patients take the place of the repeated field visits in ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
. To take a concrete example, establishing a register of children with Type 1 diabetes children were identified from hospital admission records, from general practitioners (family doctors), and from the records of the local Diabetes Association. None of these sources had a complete list, but by putting them together it was possible to do two things, first to see how many children were identified in total, and secondly to estimate how many more children with Type 1 diabetes were living in the vital community
Lincoln–Petersen method of analysis
The Lincoln–Petersen method (also known as the Lincoln indexLincoln Index
The Lincoln index is a statistical measure used in several fields to estimate the number of cases that have not yet been observed, based on two independent sets of observed cases. It is also sometimes known as the Lincoln-Petersen method.-Applications:...
) can be used to estimate population size if only two visits are made to the study area. This method assumes that the study population is "closed." In other words, the two visits to the study area are close enough in time so that no individuals die, are born, move into the study area (immigrate) or move out of the study area (emigrate) between visits. The model also assumes that no marks fall off animals between visits to the field site by the researcher, and that the researcher correctly records all marks.
Given those conditions, estimated population size is:
where
- N = Estimate of total population size
- M = Total number of animals captured and marked on the first visit
- C = Total number of animals captured on the second visit
- R = Number of animals captured on the first visit that were then recaptured on the second visit
Adaptation of the Lincoln–Petersen method
It is assumed (Krebs, 1998) that all individuals have the same probability of being captured in the second sample, regardless of whether they were previously captured in the first sample (with only two samples, this assumption cannot be tested directly).This implies that, in the second sample, the proportion of marked individuals that are caught (
) should equal the proportion of the total population that is caught (). For example, if half of the marked individuals were recaptured, it would be assumed that half of the total population was included in the second sample.In symbols,
A rearrangement of this gives
the formula used for the Lincoln–Petersen method (Krebs, 1998).
Sample calculation
A biologist wants to estimate the size of a population of turtles in a lake. She captures 10 turtles on her first visit to the lake, and marks their backs with paint. A week later she returns to the lake and captures 15 turtles. Five of these 15 turtles have paint on their backs, indicating that they are recaptured animals.
In this example, the Lincoln–Petersen method estimates that there are 30 turtles in the lake.
A refined form. The Schnabel Method of M-R
Known as the Schnabel Method of M-R.A less biased estimator
Shrinkage estimator
In statistics, a shrinkage estimator is an estimator that, either explicitly or implicitly, incorporates the effects of shrinkage. In loose terms this means that a naïve or raw estimate is improved by combining it with other information. The term relates to the notion that the improved estimate is...
of population size can be obtained with a modified version of the first formula above:
where, as before,
- N = Estimate of total population size
- M = Total number of animals captured and marked on the first visit
- C = Total number of animals captured on the second visit
- R = Number of animals captured on the first visit that were then recaptured on the second visit
An approximately unbiased variance of N, or var(N), can be estimated as:
More than two visits
The literature on the analysis of capture-recapture studies has blossomed since the early 1990s. There are very elaborate statistical models available for the analysis of these experiments. A simple model which easily accommodates the three source, or the three visit study, is to fit a Poisson regressionPoisson regression
In statistics, Poisson regression is a form of regression analysis used to model count data and contingency tables. Poisson regression assumes the response variable Y has a Poisson distribution, and assumes the logarithm of its expected value can be modeled by a linear combination of unknown...
model. Sophisticated mark-recapture models can be fit using Recapturehttp://cran.r-project.org/web/packages/Rcapture/index.html, a package of the Open Source R programming language, or specialized programs such as MARK http://www.warnercnr.colostate.edu/~gwhite/mark/mark.htm or M-SURGE http://www.cefe.cnrs.fr/biom/Logiciels.htm. The other related methods which are often used, include Jolly-Seber method (used in open populations and for multiple census estimates) and Schnabel estimates (described above as an expansion to the Lincoln-Petersen method for closed populations). These are described in detail by Sutherland.
Integrated approaches
Modeling mark-recapture data is trending towards a more integrative approach, which combines mark-recapture data with population dynamics models and other types of data. The integrated approach is more computationally demanding, but extracts more information from the data improving parameter and uncertainty estimates.See also
- German tank problemGerman tank problemIn the statistical theory of estimation, estimating the maximum of a uniform distribution is a common illustration of differences between estimation methods...
, for estimation of population size when the elements are numbered
Further reading
- Bonett, D.G., Woodward, J.A., & Bentler, P.M. (1986). "A Linear Model for Estimating the Size of a Closed Population", British Journal of Mathematical and Statistical Psychology, 39, 28-40.
- Evans, M.A., Bonett, D.G., & McDonald, L. (1994). "A General Theory for Analyzing Capture-recapture Data in Closed Populations." Biometrics, 50, 396-405.
- Lincoln, F. C. (1930). "Calculating Waterfowl Abundance on the Basis of Banding Returns". United States Department of Agriculture Circular, 118, 1–4.
- Petersen, C. G. J. (1896). "The Yearly Immigration of Young Plaice Into the Limfjord From the German Sea", Report of the Danish Biological Station (1895), 6, 5–84.
- Schofield, J. R. (2007). "Beyond Defect Removal: Latent Defect Estimation With Capture-Recapture Method", Crosstalk, August, 2007; 27-29.