# 7th Class Mathematics Statistics STATISTICS

## STATISTICS

Category : 7th Class

Learning Objectives:

• To understand the terms data, Array, Frequency, Mean, Mode and Mean.
• To understand how to find the mean of grouped and ungrouped data.
• To learn how to find the mode of given set of observation.
• To learn how to find the median of ungrouped data.
• To understand bar graphs and Histogram.
• To understand the term probability and learn how to find the probability of an event.

DATA:

A collection of numerical figures giving some particular type of information is called data.

Example: The marks obtained by 10 students in a class test (out of 50) are:

24, 43, 49, 38, 36, 36, 31, 40, 42, 15.

Raw data: Data obtained in original form is known as raw data. Data given in above example is raw data.

ARRAY:

Arranging the data in ascending or descending order is known as array. Like in above example on arranging the above data in ascending order will be as:

15, 23, 31, 36, 36, 38, 40, 42, 43, 49.

It is known as array.

TABULATION OF DATA:

Arranging the data in form of table is known as tabulation of data.

OBSERVATION:

Each numerical figure in a data is called observation.

FREQUENCY OF AN OBSERVATION:

The number of times a particular observation occurs is called its frequency. In above example 36 occurs two times so its frequency is 2.

STATISTICS:

Statistics deals with the collection, presentation, analysis and interpretation of numerical data.

MEAN OF UNGROUPED DATA:

$\text{Mean:}\,\,\frac{\text{Sum of the given observations}}{\text{No}\text{. of given observations}}$

Example: Find the mean of the numbers

7, 6, 8, 9, 5, 4, 3, 7, 8, 2

Solution: Sum of the given numbers = 7 + 6 + 8 + 9 + 5 + 4 + 3 + 7 + 8 + 2 = 59

No. of given observation = 10

$\text{Mean}=\frac{59}{10}=5.9$

MEAN OF TABULATED DATA:

If the frequency of $n$ observation ${{x}_{1}},{{x}_{2}},{{x}_{3}}........{{x}_{n}}$ are ${{f}_{1}},{{f}_{2}},{{f}_{3}}.........{{f}_{n}}$ respectively then

$\text{Mean}=\frac{({{f}_{1}}{{x}_{1}}+{{f}_{2}}{{x}_{2}}+{{f}_{3}}{{x}_{3}}.....{{f}_{x}}{{x}_{n}})}{({{f}_{1}}+{{f}_{2}}+{{f}_{3}}.....{{f}_{n}})}=\frac{\Sigma ({{f}_{i}}\times {{x}_{i}})}{\Sigma {{f}_{i}}}$

$\Sigma =$Greek letter showing summation

Example: The following table shows the weight of 15 workers in a factory.

 Weight (kg) 60 63 66 72 75 No of workers 4 5 3 1 2

Calculate the mean weight.

Solution: For calculating mean weight, we prepare following table.

 Weight (kg)${{x}_{1}}$ No. of workers ${{f}_{1}}\times {{x}_{1}}$ 60 4 240 63 5 315 66 3 198 72 1 72 75 2 150 $\Sigma {{f}_{1}}=15$ $\Sigma {{f}_{1}}{{x}_{1}}=975$

Example: Calculate the mean for the following data:

 Marks 24 25 19 15 20 22 No. of students 3 5 4 5 2 3

Solution: We may prepare the table given below:

 Marks$({{x}_{i}})$ No. of students $({{f}_{i}})$ Cumulative frequency $({{f}_{i}}\times {{x}_{i}})$ 15 5 5 75 19 4 9 76 20 2 11 40 22 3 14 66 24 3 17 72 25 5 22 125 $N=\Sigma {{f}_{i}}=22$ $\Sigma ({{f}_{i}}\times {{x}_{i}})$$=454$

Here, $N=\Sigma {{F}_{i}}=22$Which is even.

$\text{Mean}=\frac{\Sigma ({{f}_{i}}\times {{x}_{i}})}{\Sigma {{f}_{i}}}=\frac{454}{22}=20.63$

MODE

• The mode of a set of observations is the observation that occurs most often.

Putting the same observations together and counting them is not easy if the number of observations is large. In such cases we tabulate the data. Tabulation can begin by putting tally marks and finding the frequency, as we did in grouped data.

Look at the following example:

Following are the margins of victory in the football matches of a league.

1, 3, 2, 5, 1, 4, 6, 2, 5, 2, 2, 2, 4, 1, 2, 3, 1, 1, 2, 3, 2, 6, 4, 3, 2, 1, 1, 4, 2, 1, 5, 3, 3, 2, 3, 2, 4, 2, 1, 2

Find the mode of this data.

Let us put the data in a tabular form:

 Margins of victory Tally bars Number of Matches 1 $\bcancel{|\,\,|\,\,|\,\,|} \,\,|\,\,|\,\,|\,\,|$ 9 2 $\bcancel{|\,\,|\,\,|\,\,|} \bcancel{|\,\,|\,\,|\,\,|}|\,\,|\,\,|\,\,|$ 14 3 $\bcancel{|\,\,|\,\,|\,\,|}\,\,\,|\,\,|$ 7 4 $\bcancel{|\,\,|\,\,|\,\,|}$ 5 5 $|\,\,|\,\,|$ 3 6 $|\,\,|$ 2 Total 40

Looking at the table, we can quickly say that 2 is the 'mode' since 2 has occurred the highest number of times.

Thus, the most frequent victory margin is 2 goals in the league.

MEDIAN:

After arranging the given data in ascending or descending order, the value of middle-most observation is called median of the data.

MEDIAN OF UNGROUPED DATA:

Arrange the data in ascending or descending order. If the total no of observation are n.

Case 1: When $n$ is odd:

Median $=\left( \frac{n+1}{2} \right)\text{th}$observation value

Case 2: When $n$ is even

Media$=\frac{1}{2}\left\{ \frac{n}{2}th+\left( \frac{n}{2}+1 \right)\text{th}\,\text{observation}\,\text{value} \right\}$

Example: The runs scored by 1 members of cricket teams

38, 29, 54, 36, 78, 82, 59, 38, 16, 40, 60

Find the median score.

Solution: On arranging the numbers in ascending order

16, 29, 36, 38, 38, 40, 54, 59, 60, 78, 82

Here n = 11 which is odd

$\therefore$  Median score

$=\frac{1}{2}(n+1)th$term value

$=\frac{1}{2}(11+1)=\frac{1}{2}\times 12=$6th term value.

So median = 40.

Example: The weight of 8 students (in kg) are:

32, 38, 40, 30, 41, 35, 43, 30.

Find the median weight.

Solution: Arranging the weight in ascending order

30, 30, 32, 35, 38, 40, 41, 43.

Here $n=8$

$\therefore$  Median weight

$=\frac{1}{2}\left\{ \frac{\text{nth}\,\text{term}\,\text{value}}{\text{2}}+\left( \frac{n}{2}+1 \right)\text{th}\,\text{term}\,\text{value} \right\}$

$=\frac{1}{2}\left\{ \frac{8}{2}+\left( \frac{8}{2}+1 \right) \right\}$

$=\frac{1}{2}${4th term value + 5th term value}

$=\frac{1}{2}\times (35+38)=\frac{1}{2}\times 73=36.5\,kg$

Example: We have the following observations in ascending order of magnitude:

$11,\,\,12,\,\,14,\,\,18,\,\,x+2,\,\,x+4,\,\,30,\,\,32,\,\,35,\,\,41$

If the median is 24 find the value of$x.$

Solution:

Here n = 10 (even)

$\text{Median}=\frac{\frac{n}{2}\text{th observation}+\left( \frac{n}{2}+1 \right)\text{th observation}}{2}$

$24=\frac{\frac{10}{2}\text{th observation}+\left( \frac{10}{2}+1 \right)\text{th observation}}{2}$

$24=\frac{\text{5th observation + 6th observation}}{2}$

$24=\frac{(x+2)+(x+4)}{2}$

$\Rightarrow \,\,24=\frac{2x+6}{2}\Rightarrow 24=x+3$

$\Rightarrow \,\,x=21$

BAR GRAPHS

Two hundred students of 6th and 7th class were asked to name their favorite colour so as to decide upon what should be the colour of their school building. The results are shown in the following table. Represent the given data on a bar graph.

 Favourite Colour Red Green Blue Yellow Orange Number of Students 43 19 55 49 34

Answer the following questions with the help of the bar graph:

(i) Which is the most preferred colour and which is the least preferred?

(ii) How many colours are there in all? What are they?

Choose a suitable scale as follows:

Start the scale at 0. The greatest value in the data is 55, so end the scale at a value greater than 55, such as 60.

Use equal divisions along the axes, such as increments of 10. You know that all the bars would lie between 0 and 60. We choose the scale such that the length between 0 and 60 is neither too long nor too small. Here we take 1 unit for 10 students.

We then draw and label the graph as shown.

From the bar graph we conclude that

(i) Blue is the most preferred colour (Because the bar representing Blue is the tallest).

(ii) Green is the least preferred colour. (Because the bar representing Green is the shortest).

(iii) There are five colours. They are Red, Green, Blue, Yellow and Orange. (These are observed on the horizontal line)

A mathematics teacher wants to see whether the new technique of teaching she applied after the quarterly test was effective or not. She takes the scores of the 5 weakest children in the quarterly test (out of 25) and in the half yearly test (out of 25):

 Students Ashish Arun Kavish Maya Rita Quarterly 10 15 12 20 9 Half yearly 15 18 16 21 15

Since there seems to be a marked improvement in most of the students, the teacher decides that she should continue to use the new technique of teaching.

HISTOGRAM

• Histogram is used for graphical representation of a frequency distribution. A histogram is a graph that represents the class frequencies in a frequency distribution by vertical adjacent rectangles.

Example 1:

The frequency distribution of weights (in kg) of 40 persons of a locality is given.

 Weight (in kg) 40–45 45–50 50–55 55–60 60–65 Frequency 4 12 13 6 5

Solution:

Construct a histogram to represent the above data.

PROBABILITY

There are situations in our life that are certain to happen, some that are impossible and some that may or may not happen. The situation that may or may not happen not happen has chance of happening.

We know that when a coin is thrown, it was possible outcomes, Head or Tail. For a dice we have 6 possible outcomes. We also know from experience that for a coin, Head or Tail is equally likely to be obtained. We say that the probability of getting Head or Tail is equal and is$\frac{1}{2}$for each.

For a dice, probability of getting either of 1, 2, 3, 4, 5, or 6 is equal. That is, for a dice there are 6 equally likely possible outcomes. We say each of 1, 2, 3, 4, 5, 6 has one-sixth$\left( \frac{1}{6} \right)$probability.

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