Lecture/Week 2 MARB 303
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This 2 page Class Notes was uploaded by Mackenzie Thompson on Monday September 5, 2016. The Class Notes belongs to MARB 303 at Texas A&M University taught by Dr. Hui Liu in Fall 2016. Since its upload, it has received 4 views. For similar materials see Biostatistics in Ecology, Evolution, and Marine Biology at Texas A&M University.
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Date Created: 09/05/16
Lecture 2 Descriptive Statistics – Central Tendency Measures of Central Tendency All measures are concerned with capturing a single number that represents the typical case of measure of data Each measure has pros and cons and some are more appropriate than others given different levels of measurement o Pay attention to these strengths and weaknesses and to the appropriate usage Arithmetic Mean o The well known and most frequently used measure of central tendency is the mean or average (technically the arithmetic mean) Mean is the sum of the measurements divided by the number of observations Frequency Data o Frequency: counts of the number of observations in a category o Relative frequency: Frequency divided by the total number of observations in the data set o When summarizing a dataset, list all values in order Potential Issues of the mean o One of the potential drawbacks to the mean is that it can be significantly impacted by extreme values or outliers The median o The value that divides the distribution of data into 2 equal parts If there is an odd number middle observation is median If there is an even number median is the average of the 2 middle values List values in order Not impacted by outliers Mean vs. Median o Mean: Used with continuous variables Uses all the data (more information) More stable given repeated sampling More easily manipulated o Median Used with continuous or ordinal variables Unaffected by outliers Mode o The mode is the vale that occurs most frequently There can be bi-modal, tri-modal, or multi-modal distributions Can be used for all levels of measurement A distribution with only one peak (i.e. with one value occurring the most) then mode would be right under the peak Connection o For a normal, bell-shaped distribution: All measures of central tendency are the same value As the distribution diverges from normality, the values will differ o Skewed distributions: Left side is higher positively skewed Right side is higher negatively skewed
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