USMLE Biostats 1: Positive & Negative Skew (Mean, Median, Mode!) and Standard Deviation

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/ lymed Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.
Follow along with First Aid, or with my notes which can be found here:
www.dropbox.co...
This video kicks off our biostatistics block! Our first topic will be on mean, median, and mode. These are all ways to measure the central tendency. The mean is the average, found by adding all the numbers and dividing by the sample size. The median is just the middle number, so that 50 percent is below this number, and 50 percent is above it. If there are two median numbers, take the average. The mode is the most frequently occurring number. Now there are some special circumstances: in outliers, the mean is most affected by outliers. Know that if there are two modes, its called bimodal and shows up as a graph as a bimodal graph.
Now if we want to see how much a data point deviates from the average, we can calculate that as the standard deviation. Most of our data will fall within 1 SD (about 68%), 2SD will pick up 95% and finally 99% will fall within 3 SD. Our next topic is the standard error of the mean. This is the difference between the proposed mean and the actual mean. Thats the SEM and it's proportionate to sample size!
Now our last topic is on positively and negatively skewed bell curves. In a positive skewed graph, your mean is greater than your median, which is greater than your mode. Conversely, in negative skew graphs, the mode is the greatest. For more information, check out my notes. Done!

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Much love, -Mike

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how do you find the standard deviation?

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