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  1. 1. Research methodology<br />
  2. 2. contents<br />Introduction to research<br />Definitions<br /> Research<br />Thesis<br />Dissertation<br />Why to do a research<br />Categories of research<br />Research methods & research methodology<br />Procedural steps in research<br />Research strategy<br /> epidemiologic studies<br />Ethics in research<br />
  3. 3. Introduction <br />Investigation using scientific procedures, by searching again and again to find out the truth <br /> Research is the continual search for truth using the scientific method. It is the pursuit of the truth with the help of study, investigation, observation, comparison and/or experimentation.<br />
  4. 4. definitions<br />Research(WHO Health research methodology, 1992, a guide for training in research methods): <br /> Research is a quest for knowledge through diligent search or investigation or experimentation aimed at the discovery and interpretation of new knowledge. Scientific method is a systematic body of procedures and techniques applied in carrying out investigation or experimentation targeted at obtaining new knowledge. Here research and scientific methods may be considered a course of critical enquiry leading to discovery of facts or information which increases our understanding of human health and disease.<br />
  5. 5. Thesis<br />(according to Oxford English dictionary)<br /> A proposition laid down as a theme to be discussed and proved. A discussion to maintain and prove a thesis especially written or delivered by a candidate for university degree- Ph.D<br />
  6. 6. Dissertation: <br /><ul><li>Dissertation means “to discuss”.
  7. 7. It is an academic activity laid down by the university for the partial fulfillment of MDS degree.
  8. 8. It is an exercise to train a PG student to plan, execute, evaluate, write and report a scientific project.
  9. 9. It is an in-depth study of a particular topic which contributes new information and knowledge in the field
  10. 10. A formal, often lengthy treatise (a book or writing of some particular subject, one containing a methodical discussion or exposition of principles of subject) or discourse, especially one written by a candidate for a master’s degree.</li></li></ul><li>It is a pre-requisite or academic activity for acquiring a masters degree or doctorial.<br />To find out the truth which is hidden and which has not been discovered yet.<br />To contribute new knowledge to the existing one as the scientific knowledge develops in increments.<br />As a profession to conduct research in various aspects.<br />To solve an existing health problem.<br />For thrill and enjoyment of discovering a new fact.<br />
  11. 11. Categories of research<br /> Empirical researchandtheoretical research<br /> Basic researchandApplied research<br />Empirical <br />Observational research <br /><ul><li>Experimental research</li></li></ul><li>Basic or pure research<br />It is usually considered to involve the search for knowledge without a defined goal of utility or specific purpose.<br />Eg. The microbiologist, whose expertise is the identification of pathogenic micro organisms may conduct research in-vitro, in a laboratory on bacterial reproduction, growth and endotoxin production without any direct application to clinical dentistry. Probably, the researcher is interested in monitoring endotoxins produced over the life cycle of bacteria to determine a pathogenecity. <br />
  12. 12. Applied research<br />It is problem oriented and is directed towards a defined and purposeful end. <br />Eg. An oral health researcher investigating the relationship between periodontal disease and myocardial infarction may use the findings of the microbiologist as a theoretical framework to test the relationship between bacterial endotoxins, periodontal disease and myocardial infarction in animal and then in human subjects<br />
  13. 13. Research methods<br />
  14. 14. methods/ techniques that are used for conducting research are termed as research methods or research techniques.<br />They fall into 3 groups<br />Those concerned with collection of data<br />Those statistical techniques which are used for establishing relationship between the data and the unknown<br />Those methods which are used to evaluate the accuracy of the results obtained.<br />
  15. 15. Research methodology<br />It is a way to systematically solve the research problem.<br />It is necessary for the researcher to design his methodology for his problem, as the same may differ from problem to problem i.e. <br />Why a research study has been undertaken<br />How the research problem has been defined <br />In what way and why the hypothesis has been formed <br />
  16. 16. What data has been collected<br />What particular method has been adopted for collection of data<br />Why a particular technique of data collection has been used <br />And a host of similar questions are usually answered when we talk of research methodology, so that research results are capable of being evaluated either by the researcher himself or by others<br />
  17. 17. Procedural steps in research<br />
  18. 18. Selection of problem for investigation-further exploration<br />Inexperienced-seek opinion –senior<br />Constructing –essential starting point<br />Review literature<br />Modern electronic research<br />Abstract collection held on CD-ROMS-enables –key words<br />Medline and psyclit database<br />Role of systemic review grown recently-cochrane collaboration-reviews from volunteer researchers internationally adds them to a bank,which is open for inspection by interesting parties.<br />
  19. 19. Specifying study design<br />Selection of measures: many measure ,assessments <br />Eg:self report rating scales for dental anxiety,observations,interviews,stand questionnaires<br /> simple rating scale can be given a no.1-strongly agree<br /> 2-agree<br /> 3-undecided<br /> 4-disagree<br /> 5-strongly disgree<br />May not exist for the study ,this may necessiate the development of new scale.<br /> should possess Reliability and validity<br />
  20. 20. Selection of sample:<br />Sampling techniques-inorder to provide information ,can be generalised to cover the whole population.<br />Writing a protocol: aims & objectives,method,materials,sample ,procedure<br />
  21. 21. Ethical approval<br />Pilot work<br />Main study<br />Analysis of results<br />Report writing<br />publication<br />
  22. 22. Research strategies<br />Selection of a research strategy is the core of a research design and is probably the single most Important decision the investigator has to make, i.e. whether to undertake a descriptive, an analytical or an experimental study.<br />
  23. 23. Classification of Epidemiologic studies<br />
  24. 24. Decscriptive study<br />Concerned with observing the distribution of disease or health related characteristics in human population<br /><ul><li>Eg. Surveys.
  25. 25. Defining population:denominator
  26. 26. Defining the disease:operational definition.
  27. 27. Describing disease-time,place and person</li></ul>Time:<br />Short term fluctuation:<br />Common source epidemics-single,continuous<br />Propagated epidemics-person to person transmission of infectious agent---epidemics of hepatitis A and poliomyelitis<br />Slow or modern epidemics<br />
  28. 28. Periodic fluctuations:<br />Seasonal –communicable diseases like measles,varicella,malaria<br />Eg:upperResp tract infections inc during winter and GI inf in summer<br />Cyclic trends-measles-incidence every 2-3 yrs<br />Influenza occurs at intervals of 7-10 yrs<br />Long term fluctuation<br />Changes occur over long period of time,change in frequency encompassing sev decades<br />Eg:CHD,diabetes,lung cancer shows upward trend in 50 decades and TB, typhoid downward<br />Place distribution:<br />Interanational-<br />National-endemic disease like goitre,flourosis<br />Rural-urban-lung cancer,cardiovascular accidents-urban<br />Local-spot maps (john snow –on epidemic of cholera)and shaded maps -study<br />
  29. 29. Person distribution:age sex<br />Measurement of disease :morbidity and mortality indicators<br />Expressed in terms of incidence and prevalence<br />Comparing with known indices:comparision b/n different population and subgroups of same populn-clue to etiology<br />Formulation of hypothesis:relating to disease etiology<br /> specify-population,sp cause ,expected outcome,dose-response r/n,time-resp r/n.<br />Eg:smoking of 30-40 cigarettes/day causes lung cancer in 10% of smokers after 20 yrs of exposure.<br />Uses:<br />magnitude and type of disease in community<br />Clues to etiology<br />contributes to research<br />
  30. 30. surveys<br />Surveys are methods of scientific or systematic collection of data, compiling, presenting in order (useful manner), analysing and evaluating them<br />The focus of attention must be on the following<br />Formulating the objective of the study-obj describing what is to be measured,resources<br />Designing the method of data collection eg. Questionnaires, interviewing, examination of records or observations<br />Selecting the sample<br />
  31. 31. Collection of data through questionnaires<br />Questionnaire is considered as the heart of survey operation. <br />General form: <br />It can be either structured or unstructured.<br />May be either closed or open.<br />Question sequence:<br />The question sequence should be clear<br />First few questions are particularly important in seeking the desired co-operation <br />Questions related to personal character, wealth or memory of the respondent should be avoided as opening questions<br />
  32. 32. Conducting examination:<br />Obtaining approval from authorities<br />Budgeting<br />Scheduling<br />Emergency care and refferal<br />(diagnostic method)Validity and reliabilty of the data<br />
  33. 33. Classification of study types-ADA(1970)<br />typeI: complete examination<br />typeII: limited examination<br />Type III: inspection<br />Type IV:screeningprocedures:tongue depressor<br />d) Analysing the data<br />e)Drawing the conclusion<br />f) Publishing the reports<br />
  34. 34. Case study: it is an intensive investigation of a person, a family, a group, a social institution or an entire community in a natural setting<br />Correlational study: it is a descriptive research technique utilized to identify consistent relationship among variables<br />
  35. 35. Analytical study<br />Second major type<br />Subject of interest –individual with in population<br />Object –not to formulate but test hypothesis<br />Can determine:<br />1.Statistical association between disease and suspected factors<br />2.If exists--strength<br />
  36. 36. Case control studies<br /> Design of a case-control study<br />Time<br />Direction of enquiry<br />Exposed<br />Cases<br />Population<br />Not Exposed<br />Exposed<br />Controls<br />Not Exposed<br />
  37. 37. Defining the cases and controls:<br />Case: you have to decide a case before the start of the study. The case has to fit in to two criteria.<br />Diagnostic criteria.<br />Eligibility criteria.<br /> only newly diagnosed cases within a specified period of time than old cases<br />
  38. 38. Selection of cases<br />The criteria for inclusion in the study must be clearly specified. <br />Sources of cases:<br />Hospitals<br /> General population<br />
  39. 39. Controls<br />they must be as similar to the cases as possible, except for the absence of the disease,which is under study.<br />Selection of controls<br />Crucial step in case-control studies<br />Controls must be<br />Be similar to the cases except for the absence of the disease under study<br />Equal ratio<br />Sources of controls<br />Hospitals:diff illness<br />Relatives<br />Neighborhood controls<br />General population<br />
  40. 40. Matching<br />Definition: <br /> the process by which we select controls in such a way that they are similar to cases with regard to certain pertinent selected variables, which are known to influence the out come of disease and which, if not adequately matched for comparability, could distort or confound the results.<br />Example: age.<br />Confounding factor<br />One which is associated both with exposure and disease; and is distributed unequally in study and control groups<br />Although associated with ‘exposure’ under investigation, it itself is a risk factor for the disease<br />
  41. 41. Measurement of exposure<br />Exposure can be measured by<br />Interviews<br />Questionnaires<br />By studying past records<br />Examinations <br />Bias/ systematic error should be avoided while measuring the exposure<br />
  42. 42. Analysis <br />Involves two steps<br />Exposure rates among cases and controls<br />Estimation of disease risk associated with exposure (odds ratio) <br />
  43. 43. 1. Exposure rates<br />A case control study of smoking and lung cancer<br />Exposure rate among cases= (a/a+c)100 = (33/35) 100 = 94.2 % <br />Exposure rate among controls= (b/b+d)100 = (55/82) 100 = 67 % <br />
  44. 44. 2.Estimation of risk<br />Incidence among exposed <br />Relative risk = <br />Incidence among non exposed <br />Relative risk (RR) or Risk ratio<br /> = a/(a+b) / c/(c+d)<br />
  45. 45. Odds ratio (cross product ratio)<br />It is a key parameter in the analysis of case control studies<br />A measure of the strength of the association between risk factor and outcome<br />Derivation of odds ratio is based on 3 assumptions<br />Disease under investigation is a rare one<br />Cases are representative of those with disease<br />Controls are representative of those without disease<br />
  46. 46. Odds ratio = ad/bc = 33 X27/ 55X2 = 8.1<br />Smokers have a risk of having lung cancer 8.1 times that of non smokers<br />
  47. 47. Bias in case control study<br />Bias due to confounding<br />Memory bias<br />Selection bias<br />Berksoniansbias:different rates of admission to hosp for people with diff disease<br />Interviewer’s bias<br />
  48. 48. advantages of …<br />Relatively easy to carry out<br />Rapid and inexpensive (compared with cohort studies)<br />Require comparatively few subjects<br />suitable to investigate rare diseases or diseasesabout which little is known. <br />No risk to subjects<br />Allows the study of several different aetiological factors (e.g.,smoking, physical activity and personality characteristics inmyocardial infarction)<br />No attrition problems, because case control studies do notrequire follow-up of individuals into the future<br />Ethical problems minimal<br />
  49. 49. Disadvantages of …<br />High chances for bias<br />Validation of information obtained is difficult or sometimes impossible<br />Selection of an appropriate control group may be difficult<br />We cannot measure incidence, and can only estimate the odds ratio but not relative risk<br />Not suited to the evaluation of therapy or prophylaxis of a disease<br />Another major concern is the representativeness of cases and controls<br />
  50. 50. Cohort studies<br />Usually undertaken to obtain additional evidence to refute or support the existence of an association <br />between suspected cause and disease<br />Other names <br />Incidence study<br />Forward looking study<br />Longitudinal study<br />Prospective study<br />
  51. 51. distinguishing Features of …<br />Cohorts are identified prior to the appearance of the disease under investigation<br />Study groups are observed over a period of time to determine the frequency of disease<br />The study proceeds from cause to effect<br />
  52. 52. Cohort is defined as a group of people who share a common characteristic or experience within a defined time period <br />Eg, birth cohort,age cohorts, occupational cohorts, exposure to a drug cohorts, marriage cohort etc.<br />The comparison group may be…<br /> the general population from which the cohort is drawn<br />
  53. 53. Indications for …<br />When there is a good evidence of an association between exposure and disease<br />When the exposure is rare but the incidence of disease is high among exposed <br />When the attrition can be minimised<br />When ample funds are available<br />
  54. 54. Design of a cohort study<br />Time<br />Direction of enquiry<br />Screening to exclude those with the condition<br />Develop<br />Disease<br />a<br />With the <br />characteristic<br />S<br />A<br />M<br />P<br />L<br />E<br />Do not develop<br /> disease<br />Population <br />free of condition<br />b<br />Time<br />Develop<br />Disease<br />c<br />Without the <br />characteristic<br />d<br />Do not develop<br /> disease<br />
  55. 55. Considerations for selecting cohorts<br />Cohorts must be free from the disease under study<br /> both the groups should be equally susceptible to disease under study<br />Both the groups should be comparable in respect of all possible variables, except the assumed risk factors<br />Diagnostic and eligibility criteria of the disease must be defined beforehand.<br />Inclusion and exclusion criteria should be clearly stated before the commencement<br />
  56. 56. Steps in …<br />Selection of study subjects<br />Obtaining data on exposure<br />Selection of comparison groups<br />Follow up<br />Analysis<br />
  57. 57. Selection of study subjects<br />Cohorts can be selected from<br />General population<br />Special groups<br />Select groups (eg. Doctors, lawyers, teachers, etc.)<br />Exposure groups<br />
  58. 58. Obtaining data on exposure<br />Information can be obtained from <br />Cohorts <br />Review of records<br />Medical examination or special tests<br />Environmental surveys<br />Information about exposure should facilitate classification of cohort members<br />According to whether or not they were exposed<br />According to the degree of exposure<br />
  59. 59. Selection of comparison groups<br />Internal comparisons: no outside comparision group is required<br />External comparisons: when degree of exposure is not available,ext cohort .eg:smokers and non smokers,radiologists and opthamologists.<br />Comparison with general population:mortality experience of exposure group is compared with mortality experience of general populationin same geographic area<br />
  60. 60. Follow up<br />Periodic medical examination of each member<br />Reviewing physician and hospital records<br />Routine surveillence of morbidity and mortality records<br />Mailed questionnaires, telephone interviews, periodic home visits<br />
  61. 61. Analysis <br />Data is analysedinterms of <br />Incidence rates of outcome among exposed and non-exposed<br />Estimation of risk<br />Relative risk<br />Attributable risk<br />
  62. 62. Incidence rates<br />Incidence can be measured directly<br />Incidence rate among smokers = 70/7000 = 10 per thousand<br />Incidence rate among non-smokers= 3/3000 = 1 per 1000<br /> P < 0.001<br />
  63. 63. Relative risk<br />Relative risThe ratio of incidence<br /> among exposed and <br /> incidence among <br /> non-exposed<br />Also called ‘risk ratio’<br />RR=<br />RR is the direct measure of strength of association between suspected cause and effect <br />Incidence among exposed<br />Incidence among non-exposed<br />=<br />10/1 = 10<br />RR-i= no assossiation<br />RR > 1= positive assossiation<br />
  64. 64. Attributable risk<br />Incident rate among exposed – incidence rate among non-exposed<br />X 100<br />Incident rate among exposed <br />The difference in incidence rates between exposed and non-exposed groups<br />Also called risk difference<br />of lung cancer was due to their smoking<br />It indicates to what extent disease can be attributed to the exposure<br />Suggests the amount of disease that might be eliminated if the factor could be controlled<br />(10-1/10) X 100 = 90%<br />
  65. 65. Relative risk X Attributable risk <br />Relative risk<br />Etiological enquiries<br />Larger the RR, stronger the association between risk factor and outcome<br />Does not reflect the potential public health importance<br />Attributable risk<br />Gives a better idea of the impact of a successful intervention might have in reducing the problem<br />
  66. 66. Advantages of cohort studies<br />Allow the possibility of measuring directlythe relative risk of developing the condition for thosewho have the characteristic, compared to those who do not<br />Allows for a conclusion of cause-effect relationship<br />Because the presence or absence of the risk factor isrecorded before the disease occurs, there is no chanceof bias<br />
  67. 67. Cohort studies are capable of identifying other diseases that may be related to the same risk factor.<br />Unlike case-control studies, cohort studies provide thepossibility of estimating attributable risks, thus indicating the absolute magnitude of disease attributable to the risk factor.<br />
  68. 68. Disadvantages of cohort studies<br />Not always feasible.<br />Relatively inefficient for studying rare conditions.<br />They are very costly in time, personnel, space and patientfollow-up.<br />Sample sizes required for cohort studies are extremely large, especially for infrequent conditions; it is usually difficult to find and manage samples of this size.<br />The most serious problem is that of attrition, which can affect the validity of the conclusion, if it renders the samples less representative, or if the people who become unavailable are different from those actually followed up. The higher the proportion lost (say beyond 10-15%) the more serious the potential bias.<br />
  69. 69. There may also be attrition among investigators who maylose interest, leave for another job, or become involvedin another project.<br />Over a long period, many changes may occur in theenvironment, among individuals or in the type ofintervention, and these may confuse the issue ofassociation and attributable risk.<br />
  70. 70. Case control study cohort study<br />Proceeds from effect to cause<br />Starts with the disease<br />Tests whether the suspectedexposure occurs more frequentlyin those with the disease thanamong those without thedisease.<br />Involves fewer number ofsubjects<br />Yields relatively quick results<br />Suitable for the study of rarediseases<br />Generally yields only estimateof RR (odds ratio)<br />Cannot yield information aboutdiseases other than thatselected for study<br />Relatively inexpensive <br />Proceeds from "cause to effect".<br />Starts with people exposed to risk factor or suspected cause.<br />Tests whether disease occurs more frequently in those exposed, than in those not similarly exposed.<br />Involves larger number of subjects<br />Long follow-up period often needed, involving delayed results.<br />Inappropriate when the disease or exposure under investigation is rare.<br />Yields incidence rates, RR as well as AR.<br />Can yield information about more than one disease outcome.<br />Expensive.<br />
  71. 71. Experimental studies<br />Study of epidemics among colonies of experimental animals such as rats and mice .<br />AIMS<br />To provide scientific proofs of etiological factors<br />To provide a method of measuring the effectiveness and efficiency of health services <br />has all adv and disadv of cohort study and also ethics,cost and feasibility<br />Animal studies: important application<br />Advantages<br />Bred in lab,and can manipulated easily<br />They multiply rapidly <br />Disadvantages:<br />Not all human diseases can be reprodeuced<br />All conclusions – not applicable <br />
  72. 72. Human studies:<br />To investigate disease etiology and to evaluate the preventive ,therapeutic measures<br />1747-john lind-scurvy<br />1796-Edward Jennar-cowpox<br />Ethical and logistic considerations,benefits weighed againsts the risks involved<br />Volunteers –made fully aware of the experiment<br />WHO (1980)-strict code of practice<br />
  73. 73. Experimental studies<br />Randomised controlled trials<br />Non-randomized trials<br />Randomised controlled trials:<br />Involves some action,intervention or manipulation such as deliberate application or withdrawl of suspected cause.<br />Drawing up a protocal<br />Selecting reference and experimental population<br />Randomization <br />Blinding <br />Manipulation or intervention<br />Follow- up<br />Assessment of outcome<br />
  74. 74. Select population (Reference or target population)Select suitable sample (Experimental or study population)<br />D<br />E<br />s<br />i<br />G<br />N<br /> o<br />F<br /> a<br />N<br />RCT-DB<br />Selection by defined criteria<br />Potential participants<br /> (Meet selection criteria)<br />Non-participants<br />(do not meet selection criteria)<br />Non-participants<br />(do not give consent)<br />Invitation to participate<br />Participants<br />Randomization & <br />double blinding<br />Experimental group<br />Manipulation,<br />Follow up<br />& <br />Assessment <br />Control group<br />
  75. 75. Positive results: benefit of exp measure----reduced incidence or severity of disease or other appropriate outcomes of study.<br />Negative results: severity and frequency of side effects and complications ,if any death<br />Blinding:<br />Randomization cannot guard against these sorts of bias nor the size of the sample.the technique known as blinding is adopted which can be done in<br />Single blind trial:<br />here the participant is not aware whether he belongs to study group or control group. <br />Double blind trial;<br />Here neither the doctor nor the participant is aware of the group allocation and the treatment received.<br />Triple blind trial:<br />Here the participant,the investigator and the analyzer are all ‘’blind’’.<br />
  76. 76. Ethics in research<br />
  77. 77. First important code of ethics was the NURENBURG CODE of 1947. “No research could proceed in human subjects without voluntary consent.”<br />Declaration of Helsinki code of ethics on principles of ethical behaviour was established in HELSINKI-I in 1975.<br />The term “clinical research” to “biomedical research” was revised in HELSINKI- II in 1975<br />
  78. 78. institutional ethics committee or institutional review board<br />Institutions undertaking clinical trials have to constitute an Institutional Ethics Committee (IEC) or Institutional Review Board (IRB). <br />It is mandatory that each dissertation research proposal (synopsis) involving humans (irrespective of type of study, including questionnaire based surveys) or animals should be accompanied by a clearance letter from IEC.<br />
  79. 79. ethics and its principles<br />The science of ideal human character and behavior in situations where distinction must be made between right and wrong, duty must be followed and good inter-personal relations maintained.<br />When considering the ethical aspect of health research the following principles should also be kept in mind. (Helsinki declaration)<br />
  80. 80. Ethical principles<br />Non-malificence<br />Benificience<br />Veracity or truthfulness<br />Autonomy<br />Justice<br />Confidentiality <br />
  81. 81. An application should be submitted to the ethics committee in the following manner <br />Clear research objectives and rationale for undertaking the investigation in human subjects in the light of existing knowledge.<br />Subject recruitment procedures.<br />Inclusion and exclusion criteria for entry of subjects in the study.<br />Precise description of methodology of the proposed research, including intended dosage of drugs, planned duration of treatment and details of invasive procedures if any.<br />
  82. 82. A description of plans to withdraw or withhold standard therapies in the course of research.<br />The plans for statistical analysis of the study. <br />Procedure for seeking and obtaining informed consent with sample of patient information sheet and informed consent forms in English and regional languages.<br />Safety of proposed intervention and any drug or vaccine to be tested, including results of relevant laboratory and animal research.<br />
  83. 83. For research carrying more than minimal risk, an account of plans to provide medical therapy for such risk or injury or toxicity due to over dosage should be included.<br />Proposed compensation and reimbursement of incidental expenses.<br />Storage and maintenance of all data collected during the trial.<br />Plans for publication of results- positive or negative- while maintaining the privacy and confidentiality of the study participants.<br />A statement on probable ethical issues and steps taken to tackle the same. <br />
  84. 84. Considerations to be kept in mind while conducting research:<br /><ul><li>In any research on human beings, each subject must be adequately informed of the aim, methods, anticipated benefits and potential hazards of the study and the discomfort it may entail.
  85. 85. When obtaining informed consent, we should be cautious if the subject has a dependent relationship to us. No pressure or threat should be exercised.
  86. 86. In case of legal incompetence informed consent should be obtained from the legal guardian in accordance with the existing legislation.
  87. 87. Subject should be informed that they are free to abstain or to withdraw from participation at any time</li></li></ul><li>references<br /><ul><li>Behavioural sciences for dentistry. G.Humphris & M.S.Ling. Churchill Livingstone.2000.
  88. 88. Health research methodology- a guide for training in research methods. WHO 1992
  89. 89. Critical thinking; understanding and evaluating dental research. D.M.Brunette. Quintessence publishing. 1996.
  90. 90. Dentistry, dental practice and community. 5th edition. B.A. Burt & S.A.Eklund. WB Saunders company.
  91. 91. Park’s text book of preventive and social medicine. 19th edition. K.Park. Bhanot publishers. 2007.</li></li></ul><li>THANK<br />YOU<br />