With the majority of terms The normal distribution is one of the most important concepts to get to grips with. Medical statistics. Medical statistics: Describing and presenting data Main ar ticles 221 TABLE 2: Example of calculating the three measures of average on a set of numerical data Observation Age (years) 164 260 359 454 542Median (middle) age = 42 years 635 724Modal (most common) age = 24 years 824 918 Total 380 Mean = 380 / 9 = 42.2 years MSC 2(4) 11/22/04 4:54 PM Page 221 We can, therefore, say with confidence that the effect we have observed is statistically significant, and are thus inclined to reject the null hypothesis. The chi-squared test returns a statistic – the corresponding p-value for this statistic can then be found on a table. A comprehensive collection of clinical examination OSCE guides that include step-by-step images of key steps, video demonstrations and PDF mark schemes. endstream endobj startxref Put another way, the probability of progression for an individual in the ocrelizumab group at any given time is 76% that of the placebo group. This is where the distribution is not symmetrical and has a more prominent tail pointing in one direction. from some of the patients in the hospital (our sample) to all of them (our population of interest)). The best statistical methods to use vary depending on the type of variables in question. Let’s now apply the information in table 6 to a theoretical population of 5,000 stroke patients. Regression assesses the relationship between the mean value of one variable and the deviation from this by other variables. 68% of the area under the curve is within 1 standard deviation either side of the mean. First, we calculate the proportion of patients in the exposed and unexposed groups (exposed = and unexposed = ). A sample of such a table can be found below (table 1). If the sample size is smaller, the sampling distribution of the mean will only be normal if the actual distribution in the population is normal. Standard deviation is used to express variation in the data of our sample; standard error should be used to describe the precision in the sample mean (i.e. Let’s see an example. In order to collect and analyse data appropriately, the variables involved must first be classified. 3916 0 obj <> endobj In order to collect and analyse data appropriately, the variables involved must first be classified. Essentials of medical statistics, 2nd ed. The equation for calculating the correlation coefficient in a parametric data set is below: This can look far more daunting than it really is. We have two groups (an exposure and a control) and two mean outcome values (one for each group). Risk ratios (or relative risks) are one way of comparing two proportions. NIST/SEMATECH e-Handbook of Statistical Methods. Graunt, J. Chi-squared tests (or Pearson’s chi-squared tests) are applied to establish whether there is a significant difference between two groups of categorical data. For example, we may observe that as ice cream sales increase, so do the number of drownings. 3922 0 obj <>/Filter/FlateDecode/ID[<0ED45C985475814D8708EC97A01BC128>]/Index[3916 17]/Info 3915 0 R/Length 52/Prev 1170819/Root 3917 0 R/Size 3933/Type/XRef/W[1 2 1]>>stream The equation for which is below. Ppt download. The data exhibited in life tables can be used to construct survival curves. The z-score is the number of standard deviations away from the mean that the mean difference lies (or the value on the x-axis on the graph of the standard normal distribution (see the section on SND)). Elaborate course: medical statistics. Any value on the x-axis of any normal distribution can be converted to a corresponding x-axis value on the standard normal distribution (a z-value) with the following equation: You would use this equation if you wanted to find out what percentage of the data lay beyond a certain point or within a particular range on the curve of your normal data set. The percentage of tumour volume reduction can then be derived from this. Medical Statistics STATISTICAL ANALYSIS useful INFORMATION In medical field DATA (accuracy) 6. Check out our brand new medical MCQ quiz platform at https://geekyquiz.com. It is widely recommend on a variety of courses and programmes, from undergraduate medicine, through to professional medical qualifications. Typically, two-sided p-values are used in analyses – this means that the assessment is of the size of the difference to the null hypothesis, not the direction in which that difference is (above or below the mean). Another distortion that may be present in the data is kurtosis. Statistically Figuring Sample Size When designing a study, the sample size is an important consideration because the larger the sample size, the more data you have, and the more precise your results will be (assuming high-quality data). It is calculated with the following equation: Here is an example observed data set (table 3): From this, we can find what the expected values would be (table 4). When looking for critical values in a table, you will often have to find the row with the appropriate number of degrees of freedom. Petrie A, Sabin C. Medical statistics at a glance. If we were to take lots of samples, it is not likely that the sample statistics for each sample would be the same (this is due to sampling variation); however, there should be a tendency towards the true value for the population. (1662) Natural and political observations men Skewed distributions image. In our example, the outcome was a polio diagnosis. Other general symbols and abbreviations are defined below also. h�bbd``b`~$��% �~$vH|4b`b�d100C���� � Wd� It is not necessary to know how to do a statistical analysis to critically appraise a paper. Normal distributions can be entirely defined in terms of the mean and standard deviation in the data set. To understand them, we must first construct the table below (Table 8). Offr, 55, 217. ' Medical Statistics Made Easy 3rd Edition by Michael Harris (Author), Jacquelyn Taylor (Author) 4.4 out of 5 stars 147 ratings. the z-score for an 80% CI = 1.282; the z-score for a 95% CI = 1.960). This is because ice cream sales will increase in the summer; simultaneously more people will be going for a swim. Number needed to treat (NNT) is a popular way of demonstrating the effectiveness of a treatment. When r = 1 there is a perfect positive correlation – when r = -1 there is a perfect negative correlation. Available from: [. In a population of 5,000, this means 500 lives will be saved (Table 6). New York: McGraw-Hill Education; 2016. the mean of the sampling distribution of the mean). Let’s say our z-score was 1.96. MEDICAL STATISTICSfrom A to Z From ‘Abortion rate’ to ‘Zygosity determination’, this accessible introduction to the terminology of medical statistics describes more than 1500 terms, all clearly explained, illustrated and deﬁned in non-technical language, without any mathematical formulae! Normal diet: 17/303 had heart attacks = 5.61% Mediterranean diet: 5/302 had heart attacks: 1.67% 1.67 / 5.61 = 29.5%. A collection of surgery revision notes covering key surgical topics. The corresponding test for nonparametric data is also given. This corresponds to an area under the SND curve of 0.025. Note that it is acceptable to record expected values with decimal points. Basic statistics. Odds ratios are most commonly used in gambling and are calculated with the equation below: In our example in table 1, the odds ratio would be calculated as below: Whereas probabilities can only adopt a value between 0 and 1, odds may have any value from 0 to infinity. Available from: [. Below are summarised some key features of a normal distribution (figure 1): A normal distribution with a mean of 0 and a standard deviation of 1 is called the standard normal distribution (SND) and has particular importance in statistics (figure 2). The LR for a positive result is the chance of a true positive versus that of a false positive. A collection of communication skills guides, for common OSCE scenarios, including history taking and information giving. There are numerous types of bias to be aware of, some of which are listed below: Confounding variables are those which are linked to the outcome but have not been accounted for in the study. Hypothesis tests allow us to establish the likelihood that the association we are observing is genuine, or simply due to chance. Whereas large amounts of small deviations from the mean result in platykurtosis. A correlation coefficient (r) can be calculated to show how well these two variables are associated and can have a value between -1 and 1. the effect of smoking on GFR). This is often not a problem until you undertake a research project or an intercalated degree when you suddenly realise how useful just a little knowledge on the subject can be. We encountered such a graph (a Kaplan-Meier curve) when discussing hazard ratios (figure 5). Although the normal distribution is very common in medical statistics, it is not the only way in which data can be distributed. Medical PowerPoint Presentation Design Slides. One can either have this diagnosis or not – there are two possible outcomes. OTE/SPH OTE/SPH JWBK220-FM November 28, 2007 11:13 Char Count= 0 Medical Statistics from Scratch Second Edition i. Below is a graph from a paper looking at the effect of the humanised monoclonal antibody ocrelizumab on the progression of multiple sclerosis (figure 5) (1). This makes perfect sense as we would expect 68.26% of the data to lie within 1 standard deviation either side of the mean. Medical Statistics from Scratch An Introduction for Health Professionals Second Edition David Bowers Honorary Lecturer, School of Medicine, University of Leeds, UK iii. Just as we may calculate the mean and standard deviation for one of our samples, calculating the standard error of the mean is the standard deviation for our sampling distribution of the mean. As 5% is equal to 0.05 as a decimal, our p-value is 0.05 (which by conventional standards is the threshold for statistical significance). 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