b. Explain in your own words the possible impact of publication bias in a study
ID: 3155734 • Letter: B
Question
b. Explain in your own words the possible impact of publication bias in a study of this sort.
In 2012, Dimetri and colleagues published a paper on the safety and effectiveness of MMR vaccines. Part of the abstract from that paper is included below. Background: Mumps, measles and rubella (MMR) arc serious diseases that can lead to potentially fatal illness, disability and death. However, public debate over the safety of the trivalent MMR vaccine and the resultant drop in vaccination coverage in several countries persists, despite its almost universal use and accepted effectiveness. Objectives: To assess the effectiveness and adverse effects associated with the MMR vaccine in children up to 15 years of age. Search methods: for this update we searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 2011, Issue 2), which includes the Cochrane Acute Respirators- Infections Group rsquo s Specialized Register, PubMed (July 2004 to May week 2, 2011) and Lmbasc.com (July 2004 to May 2011). Selection criteria: We used comparative prospective or retrospective trials assessing the effects of the MMR vaccine compared to placebo, do nothing or a combination of measles, mumps and rubella antigens on healthy individuals up to 15 years of age. Main results: We included five randomized controlled trials (RCTs), one controlled clinical trial (CCT), 27 cohort studies, 17 case-control studies, five time-senes trials, one ease cross-over trial, two ecological studies, six self controlled ease series studies involving in all about 14,700,000 children and assessing effectiveness and safety of MMR vaccine. Based on the available evidence, one MMR vaccine dose is at least 95% effective in preventing clinical measles and 92% effective in preventing secondary eases among household contacts... Exposure to the MMR vaccine was unlikely to be associated with autism, asthma, leukaemia, hay fever, type I diabetes, gait disturbance, Crohn rsquo s disease. Notes from the Cochrane definitions: ldquo If the author(s) state explicitly (usually by some variant of the term lsquo random rsquo to describe the allocation procedure used) that the groups compared in the trial were established by random allocation, then the trial is classified as a RCT (randomized controlled trial). If the author(s) do not state explicitly that the trial was randomized, but randomization cannot be ruled out, the report is classified as a CCT (controlled clinical trial). The classification CCT is also applied to quasi-randomized studies, where the method of allocation is known but is not considered strictly random, and possibly quasi-randomized trials. Examples of quasi-random methods of assignment include alternation, date of birth, and medical record number. rdquo Time series trial: ldquo A study that uses observations at multiple time points before and after an intervention... T he design attempts to detect whether the intervention has had an effect significantly greater than any underlying trend over time. rdquo How would you describe the design of this study? Cohort study Meta-analysis Randomized trial Case report/ Case seriesExplanation / Answer
The objective of the study was to acess the effective ness and adverse effect of MMR vaccine in childern upto age of 15 years.
(ii) It can not be a meta analysis as in meta analysis there is a common truth behind all conceptually similar scientific studies, but which has been measured with a certain error within individual studies.
(iii) The people participating in the randomnised trial are randomly allocated to either the group receiving the treatment under investigation or to a group receiving standard treatment (or placebo treatment) as the control. No such alllocation or involvement was shown in this study so it can not be randomnised trail.
(iv) IN case reports/ case series studies detailed report of the symptoms, signs, diagnosis, treatment, and follow-up of an individual patient and tracks of subjects with a known exposure, such as patients who have received a similar treatment,[1] or examines their medical records for exposure and outcome. So the present study can not be the case report/case series study.
Analysing the selection criterion we find that this study can be cohot study as in chorot study there is a passive follow-up of a group of people and a documentation of relevant characteristics or events related to this group of people. Similar to this methodology was followed in the reported study (see in search methods) the aouthors searched Cochrane central register of controlled trails.
So I think this study is Cohort study.
(B)
Actually Publication bias is a type of bias with regard to what academic research is likely to be published, among what is available to be published. More or less like a publications for forced interpetation.
In cohort study the possible bias can bias in selection of all the studies that have been conducted. That mean sleceting the desired studies among the all avialable studies.
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