Show The basic measures of central tendency are mean, median, and mode. Given a collection of data, a common question is about where the data resides. Knowing the center or mid-point or average is a starting point as we consider the data. Keep in mind that knowing the average of the data is not sufficient to make very many conclusions. Also, consider the spread or variance of the data prior to making decisions. The mean, $- \bar{X} -$, or what we commonly call the average is the total of all the data values divided by the number of data points. $$ \large\displaystyle \bar{X}=\frac{\sum\nolimits_{i=1}^{n}{{{x}_{i}}}}{n}$$ xi is the
individual data values The mean is the first moment of the data. It is the center of mass. If the data were weights along a ruler, the mean would be the balance point with an equal amount of weight and distance from the mean on both sides. The mean is a very common measure of central tendency. The mean defines the center of gravity (mass) of the data, it uses all the data, and no sorting is required. On the other hand, extreme values may distort where the bulk of the values exist, it may be more time consuming to calculation than median or mode, and it is possible the mean is not actually the value of any of the data points. The MedianThe median, $- \tilde{X} -$, is the middle value when the data is sorted in ascending or descending order. For an uneven number of values, the median is the middle value. For an even number of values, the median is the average of the two middle values. Here are two examples of sorted data and in both cases, 5 is the median. With 9 values, 1, 3, 4, 4, 5, 7, 8, 9, 9 Where the 5 is the midpoint of the sorted data, with a count of four values on either side. And with 10 values 1, 3, 4, 4, 4, 6, 7, 8, 9, 9 Where 4 and 6 are the two middle points, with an average of 5. The median provides information on where most of the data lies, thus is not sensitive to extreme values, and it requires little calculation other than sorting. The median calculation does require sorting, which may be tedious for large data sets. If the dataset does have extreme values they may be important and are ignored by the median. It is not meaningful to average medians to determine a combined data set median. The median will vary more from sample to sample than the mean. The ModeThe mode is the most frequently occurring value in the dataset. Note it is possible for a dataset to have more than one mode. In this dataset 1, 3, 4, 4, 4, 6, 7, 8, 9, 9 There are three 4’s, more than any other value, thus the mode is 4. To determine the mode, simply identify the value that occurs the most often. If there is a tie then the dataset has more than one mode. The mode I no influenced by extreme values or outliers, and it is an actual value. The mode is easy to identify with a histogram type plot or similar graphic. The mode may or may not be near the mean or median and there may be more than one mode. Related: Statistical Terms (article) Role of reliability statistics (article) Statistical Terms about Variation (article) This section focuses on measures of central tendency. Many times you are asking what to expect on average. Such as when you pick a major, you would probably ask how much you expect to earn in that field. If you are thinking of relocating to a new town, you might ask how much you can expect to pay for housing. If you are planting vegetables in the spring, you might want to know how long it will be until you can harvest. These questions, and many more, can be answered by
knowing the center of the data set. There are three measures of the “center” of the data. They are the mode, median, and mean. Any of the values can be referred to as the “average.” There are no symbols for the mode and the median, but the mean is used a great deal, and statisticians gave it a symbol. There are actually two symbols, one for the population parameter and one
for the sample statistic. In most cases you cannot find the population parameter, so you use the sample statistic to estimate the population parameter. Definition \(\PageIndex{1}\): Population Mean The population mean is given by \(\mu=\dfrac{\sum x}{N}\), pronounced mu where Definition \(\PageIndex{2}\): Sample Mean Sample Mean: \(\overline{x}=\dfrac{\sum x}{n}\), pronounced x bar, where
The value for \(\overline{x}\) is used to estimate \(\mu\) since \(\mu\) can't be calculated in most situations. Example \(\PageIndex{1}\) finding the mean, median, and mode Suppose a vet wants to find the average weight of cats. The weights (in pounds) of five cats are in Example \(\PageIndex{1}\). Table \(\PageIndex{1}\): Finding the Mean, Median, and Mode
Find the mean, median, and mode of the weight of a cat. Solution Before starting any mathematics problem, it is always a good idea to define the unknown in the problem. In this case, you want to define the variable. The symbol for the variable is \(x\). The variable is \(x =\) weight of a cat Mean: \(\overline{x}=\dfrac{6.8+8.2+7.5+9.4+8.2}{5}=\dfrac{40.1}{5}=8.02\) pounds Median: You need to sort the list for both the median and mode. The sorted list is in Example \(\PageIndex{2}\). Table \(\PageIndex{2}\): Sorted List of Cat's Weights
There are 5 data points so the middle of the list would be the 3rd number. (Just put a finger at each end of the list and move them toward the center one number at a time. Where your fingers meet is the median.) Table \(\PageIndex{3}\): Sorted List of Cats' Weights with Median Marked
The median is therefore 8.2 pounds. Mode: This is easiest to do from the sorted list that is in Example \(\PageIndex{2}\). Which value appears the most number of times? The number 8.2 appears twice, while all other numbers appear once. Mode = 8.2 pounds. A data set can have more than one mode. If there is a tie between two values for the most number of times then both values are the mode and the data is called bimodal (two modes). If every data point occurs the same number of times, there is no mode. If there are more than two numbers that appear the most times, then usually there is no mode. In Example \(\PageIndex{1}\), there were an odd number of data points. In that case, the median was just the middle number. What happens if there is an even number of data points? What would you do? Example \(\PageIndex{2}\) finding the median with an even number of data points Suppose a vet wants to find the median weight of cats. The weights (in pounds) of six cats are in Example \(\PageIndex{4}\). Find the median. Table \(\PageIndex{4}\): Weights of Six Cats
Solution Variable: \(x =\) weight of a cat First sort the list if it is not already sorted. There are 6 numbers in the list so the number in the middle is between the 3rd and 4th number. Use your fingers starting at each end of the list in Example \(\PageIndex{5}\) and move toward the center until they meet. There are two numbers there. Table \(\PageIndex{5}\): Sorted List of Weights of Six Cats
To find the median, just average the two numbers. median \(=\dfrac{7.5+8.2}{2}=7.85\) pounds The median is 7.85 pounds. Example \(\PageIndex{3}\) finding mean and median using technology Suppose a vet wants to find the median weight of cats. The weights (in pounds) of six cats are in Example \(\PageIndex{4}\). Find the median Solution Variable: \(x=\) weight of a cat You can do the calculations for the mean and median using the technology. The procedure for calculating the sample mean ( \(\overline{x}\) ) and the sample median (Med) on the TI-83/84 is in Figures 3.1.1 through 3.1.4. First you need to go into the STAT menu, and then Edit. This will allow you to type in your data (see Figure \(\PageIndex{1}\)). Figure \(\PageIndex{1}\): TI-83/84 Calculator Edit SetupOnce you have the data into the calculator, you then go back to the STAT menu, move over to CALC, and then choose 1-Var Stats (see Figure \(\PageIndex{2}\)). The calculator will now put 1-Var Stats on the main screen. Now type in L1 (2nd button and 1) and then press ENTER. (Note if you have the newer operating system on the TI-84, then the procedure is slightly different.) If you press the down arrow, you will see the rest of the output from the calculator. The results from the calculator are in Figure \(\PageIndex{3}\). Figure \(\PageIndex{2}\): TI-83/84 Calculator CALC Menu Figure \(\PageIndex{3}\): TI-83/84 Calculator Input for Example \(\PageIndex{3}\) VariableFigure \(\PageIndex{4}\): TI-83/84 Calculator Results for Example \(\PageIndex{3}\) VariableThe commands for finding the mean and median using R are as follows: variable<-c(type in your data with commas in between) So for this example, the commands would be weights<-c(6.8, 8.2, 7.5, 9.4, 8.2, 6.3) Example \(\PageIndex{4}\) affect of extreme values on mean and median Suppose you have the same set of cats from Example \(\PageIndex{1}\) but one additional cat was added to the data set. Example \(\PageIndex{6}\) contains the six cats’ weights, in pounds. Table \(\PageIndex{6}\): Weights of Six Cats
Find the mean and the median. Solution Variable: \(x=\) weight of a cat mean \(=\overline{x}=\dfrac{6.8+7.5+8.2+8.2+9.4+22.1}{6}=10.37\) pounds The data is already in order, thus the median is between 8.2 and 8.2. median \(=\dfrac{8.2+8.2}{2}=8.2\) pounds The mean is much higher than the median. Why is this? Notice that when the value of 22.1 was added, the mean went from 8.02 to 10.37, but the median did not change at all. This is because the mean is affected by extreme values, while the median is not. The very heavy cat brought the mean weight up. In this case, the median is a much better measure of the center. An outlier is a data value that is very different from the rest of the data. It can be really high or really low. Extreme values may be an outlier if the extreme value is far enough from the center. In Example \(\PageIndex{4}\), the data value 22.1 pounds is an extreme value and it may be an outlier. If there are extreme values in the data, the median is a better measure of the center than the mean. If there are no extreme values, the mean and the median will be similar so most people use the mean. The mean is not a resistant measure because it is affected by extreme values. The median and the mode are resistant measures because they are not affected by extreme values. As a consumer you need to be aware that people choose the measure of center that best supports their claim. When you read an article in the newspaper and it talks about the “average” it usually means the mean but sometimes it refers to the median. Some articles will use the word “median” instead of “average” to be more specific. If you need to make an important decision and the information says “average”, it would be wise to ask if the “average” is the mean or the median before you decide. As an example, suppose that a company wants to use the mean salary as the average salary for the company. This is because the high salaries of the administration will pull the mean higher. The company can say that the employees are paid well because the average is high. However, the employees want to use the median since it discounts the extreme values of the administration and will give a lower value of the average. This will make the salaries seem lower and that a raise is in order. Why use the mean instead of the median? The reason is because when multiple samples are taken from the same population, the sample means tend to be more consistent than other measures of the center. The sample mean is the more reliable measure of center. To understand how the different measures of center related to skewed or symmetric distributions, see Figure \(\PageIndex{5}\). As you can see sometimes the mean is smaller than the median and mode, sometimes the mean is larger than the median and mode, and sometimes they are the same values. Figure \(\PageIndex{5}\): Mean, Median, Mode as Related to a DistributionOne last type of average is a weighted average. Weighted averages are used quite often in real life. Some teachers use them in calculating your grade in the course, or your grade on a project. Some employers use them in employee evaluations. The idea is that some activities are more important than others. As an example, a fulltime teacher at a community college may be evaluated on their service to the college, their service to the community, whether their paperwork is turned in on time, and their teaching. However, teaching is much more important than whether their paperwork is turned in on time. When the evaluation is completed, more weight needs to be given to the teaching and less to the paperwork. This is a weighted average. Definition \(\PageIndex{3}\) Weighted Average \(\dfrac{\sum x w}{\sum w}\) where \(w\) is the weight of the data value, \(x\). Example \(\PageIndex{5}\) weighted average In your biology class, your final grade is based on several things: a lab score, scores on two major tests, and your score on the final exam. There are 100 points available for each score. The lab score is worth 15% of the course, the two exams are worth 25% of the course each, and the final exam is worth 35% of the course. Suppose you earned scores of 95 on the labs, 83 and 76 on the two exams, and 84 on the final exam. Compute your weighted average for the course. Solution Variable: \(x=\) score The weighted average is \(\dfrac{\sum x w}{\sum w}=\dfrac{\text { sum of the scores times their weights }}{\text { sum of all the weights }}\) weighted average \(=\dfrac{95(0.15)+83(0.25)+76(0.25)+84(0.35)}{0.15+0.25+0.25+0.35}=\dfrac{83.4}{1.00}=83.4 \%\) A weighted average can be found using technology. The procedure for calculating the weighted average on the TI-83/84 is in Figures 3.1.6 through 3.1.9. First you need to go into the STAT menu, and then Edit. This will allow you to type in the scores into L1 and the weights into L2 (see Figure \(\PageIndex{6}\)). Figure \(\PageIndex{6}\): TI-3/84 Calculator Edit SetupOnce you have the data into the calculator, you then go back to the STAT menu, move over to CALC, and then choose 1-Var Stats (see Figure \(\PageIndex{7}\)). The calculator will now put 1-Var Stats on the main screen. Now type in L1 (2nd button and 1), then a comma (button above the 7 button), and then L2 (2nd button and 2) and then press ENTER. (Note if you have the newer operating system on the TI-84, then the procedure is slightly different.) The results from the calculator are in Figure \(\PageIndex{9}\). The \(\overline{x}\) is the weighted average. Figure \(\PageIndex{7}\): TI-83/84 Calculator CALC MenuFigure \(\PageIndex{8}\): TI-83/84 Calculator Input for Weighted AverageFigure \(\PageIndex{9}\): TI-83/84 Calculator Results for Weighted AverageThe commands for finding the mean and median using R are as follows: x<-c(type in your data with commas in between) So for this example, the commands would be x<-c(95, 83, 76, 84) Example \(\PageIndex{6}\) weighted average The faculty evaluation process at John Jingle University rates a faculty member on the following activities: teaching, publishing, committee service, community service, and submitting paperwork in a timely manner. The process involves reviewing student evaluations, peer evaluations, and supervisor evaluation for each teacher and awarding him/her a score on a scale from 1 to 10 (with 10 being the best). The weights for each activity are 20 for teaching, 18 for publishing, 6 for committee service, 4 for community service, and 2 for paperwork.
Solution a. Variable: \(x=\) rating The weighted average is \(\dfrac{\sum x w}{\sum w}=\dfrac{\text { sum of the scores times their weights }}{\text { sum of all the weights }}\) evaluation \(=\dfrac{8(20)+9(18)+2(6)+1(4)+8(2)}{20+18+6+4+2}=\dfrac{354}{50}=7.08\) b. evaluation \(=\dfrac{6(20)+8(18)+9(6)+10(4)+10(2)}{20+18+6+4+2}=\dfrac{378}{50}=7.56\) c. The second faculty member has a higher average evaluation. You can find a weighted average using technology. The last thing to mention is which average is used on which type of data. Mode can be found on nominal, ordinal, interval, and ratio data, since the mode is just the data value that occurs most often. You are just counting the data values. Median can be found on ordinal, interval, and ratio data, since you need to put the data in order. As long as there is order to the data you can find the median. Mean can be found on interval and ratio data, since you must have numbers to add together. HomeworkExercise \(\PageIndex{1}\)
1. mean = 253.93, median = 268, mode = none 3. mean = 67.68 km, median = 64 km, mode = 56 and 64 km 5. a. mean = $89,370.42, median = $75,311, b. mean = $79,196.56, median = $74,773, c. See solutions, d. See solutions, e. See solutions 7. a. ordinal- median and mode, b. ratio – all three, c. interval – all three, d. nominal – mode 9. Skewed right, mean higher 11. 2.71 13. 76.75 Which measure of center is the most frequently occurring value in a data set?Measures of central tendency help you find the middle, or the average, of a data set. The 3 most common measures of central tendency are the mean, median and mode. The mode is the most frequent value. The median is the middle number in an ordered data set.
What is a measure of center?Measures of Center and Spread
Recall that a measure of center, or central tendency, is a single number used to describe a set of numeric data. It describes a typical value within the data set. The mean and median are the two most common measures of center. The mean is often called the average.
What is measure of center in statistics?There are three measures of the “center” of the data. They are the mode, median, and mean. Any of the values can be referred to as the “average.” The mode is the data value that occurs the most frequently in the data.
What is the most commonly used measure of the center of data and it is also referred as the arithmetic average?Mean (Arithmetic)
The mean (or average) is the most popular and well known measure of central tendency. It can be used with both discrete and continuous data, although its use is most often with continuous data (see our Types of Variable guide for data types).
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