MDM_bias_sampling

Luka, Mary **Sampling Bias** Sampling bias means that the statistics that are collected are selected incorrectly and do not represent the true distribution because of non-random reasons. There are different types of sampling bias; selection from a specific area, self-selection, pre-screening, and exclusion.  Selection from a specific area is self-explanatory. For example if a survey is asking if Toronto is a clean city, only people from Toronto would be surveyed. This is bias because only the people from Toronto are being surveyed, therefore only a specific area is being surveyed and the results will be incomplete. The reason it is incomplete is because the survey does not reach a wider range of opinions.  Self-selection is when there is a group of people being surveyed and some of the members of the group choose not to complete the survey. For example, if a group of high school students are asked whether or not they think homework is useful, the students that have a lower average may not want to answer the survey because they are ashamed, when the smarter students would be more likely to participate.  Pre-screening or advertising is when the surveyer is on both sides of the subject which he is using, for example: the surveyer is trying to prove that smoking does not affect a person’s health but only asks people that smoke in aerobics and non smokers in weight loss classes.  Exclusion bias is when a surveyer does not have the complete result because some of the members are not available, or drop out of the survey during any point that it is being asked.  A bias sample can cause problems because  A biased sample causes problems because any statistic computed from that sample has the potential to be consistently erroneous. The bias can lead to an over- or under-representation of the corresponding parameter in the population. Almost every sample in practice is biased because it is practically impossible to ensure a perfectly random sample. If the degree of under-representation is small, the sample can be treated as a reasonable approximation to a random sample. Also, if the group that is under-represented does not differ markedly from the other groups in the quantity being measured, then a random sample can still be a reasonable approximation. The word bias in common usage has a strong negative word connotation, and implies a deliberate intent to mislead or other scientific fraud. In statistical usage, bias merely represents a mathematical property, no matter if it is deliberate or either unconscious or due to imperfections in the instruments used for observation. While some individuals might deliberately use a biased sample to produce misleading results, more often, a biased sample is just a reflection of the difficulty in obtaining a truly representative sample. Some samples use a biased statistical design which nevertheless allows the estimation of parameters. The U.S. National Center for Health Statistics. for example, deliberately oversamples from minority populations in many of its nationwide surveys in order to gain sufficient precision for estimates within these groups These surveys require the use of sample weights (see below) to produce proper estimates across all racial and ethnic groups. Provided that certain conditions are met (chiefly that the sample is drawn randomly from the entire sample) these samples permit accurate estimation of population parameters. Refrences


 * 1) **[|^] ** <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|TheFreeDictionary--> biased sample] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> Retrieved on 2009-09-23. Site in turn cites: Mosby's Medical Dictionary, 8th edition. <span style="font-family: 'Times New Roman',Times,serif;">
 * 2) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|Medical Dictionary - 'Sampling Bias'] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> Retrieved on September 23, 2009
 * 3) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|Dictionary of Cancer Terms --> Selection Bias] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> Retrieved on September 23, 2009
 * 4) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|The effects of sample selection bias on racial differences in child abuse reporting] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> Ards S, Chung C, Myers SL Jr. Child Abuse Negl. 1999 Dec;23(12):1209; author reply 1211-5. PMID: 9504213
 * 5) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|Sample Selection Bias Correction Theory] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> Corinna Cortes, Mehryar Mohri, Michael Riley, and Afshin Rostamizadeh. New York University.
 * 6) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|Page 262 in: Behavioral Science. Board Review Series.] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> By Barbara Fadem. ISBN: 0781782570, 9780781782579. 216 pages
 * 7) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|National Center for Health Statistics (2007). Minority Health.] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;">
 * 8) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> " <span style="font-family: 'Times New Roman',Times,serif;">[|Browser Statistics]  <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;">". Refsnes Data. June 2008. <span style="font-family: 'Times New Roman',Times,serif;">[]  <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;">. Retrieved 2008-07-05.     <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;">
 * 9) **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[|^] **<span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> based on <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt; mso-ansi-language: EN; msoansilanguage: EN;">[] <span style="font-family: 'Times New Roman',Times,serif; font-size: 11pt;"> retrieved on September 29, 2007