PROBABILITY

Chapter -13

PROBABILITY

Introduction

In everyday life, we come across statement such as

•  It will probably rain today.
• I doubt that he will pass the test.
• Most probably. Kavita will stand first in the annual examination.
• Chances are high that the prices of diesel will go up.
• There is a 50 – 50 chance of India winning a toss in today’s match.

The word ‘probably’, ‘doubt’, ‘most probably’, ‘chance’, etc., used in the statement above involve an element of uncertainty. For example, in (1), ‘probably rain’ will mean it may not rain today. We are predicting rain today based on our past experience when it rained under similar conditions. Similar predictions are also made in other cases listed in (2) to (5).
The uncertainty of ‘probably’ etc can be measured numerically by means of ‘probability’ in many cases.
Though probability started with gambling, it has been used extensively in the fields of physical Sciences, Commerce, Biological Sciences, Medical Sciences, Weather Forecasting, etc.

Probability – an Experimental Approach

In earlier classes, you have had a glimpse of probability when you performed experiments like tossing of coins, throwing of dice, etc,. and observed their outcomes. You will now learn to measure the chance of occurrence of a particular outcome in an experiment.

Write down the value of the following fractions:

And 

       

Toss the coin twenty times and in the same way record your observations as above. Again find the values of the fractions given above for this collection of observations.
Repeat the same experiment by increasing the number of tosses and record the number of heads and tails. Then find the values of the corresponding fractions.
You will find that as the number of tosses gets larger, the values of the fractions come closer to 0.5. To record what happens in more tosses, the following group activity can also be performed:

Find the values of the following fractions:

    

 

   

(ii) Now throw the die 40 times, record the observations and calculate the fractions as done in (i). 
As the number of throws of the die increases, you will find that the value of each fraction calculated in (1) and (2) comes closer and closer to 1/6.    
To see this, you could perform a group activity, as done in Activity 2. Divide the student in your class, into small groups. One student in each group should throw a die ten times. Observations should be noted and cumulative fractions should be calculated.
The values of the fractions for the number 1 can be recorded in Table. This table can be extended to write down fractions for the other numbers also or other tables of the same kind can be created for the other numbers. 

Write down the fractions:                  

 

                  

 

Calculate the values of these fractions.
Now, increases the number of tosses (as in Activity 2). You will find that the more the number of tosses, the closer are the values of A, B and C to 0.25, 0.5 and 0.25, respectively. 
In Activity 1, each toss of a coin is called a trial. Similarly in Activity 3, each throw of a die is a trial, and each simultaneous toss of two coin in Activity 4 is also a trial.
So, a trial is an action which results in one or several outcomes. The possible outcomes in Activity 1 were Head and Tail; whereas in Activity 1 where Head and Tail; whereas in Activity 3, the possible outcomes were 1, 2, 3, 4, 5 and 6.
In Activity 1, the getting of a head in a particular throw is an event with outcome ‘head’. Similarly, getting a tail is an event with outcome ‘tail’. In Activity 2, the getting of a particular number, say 1, is and event with outcome 1.  
If your experiment was to throw the die for getting an even number, then the event would consist of three outcomes, namely, 2, 4 and 6.
So, an event for an experiment is the collection of some outcomes of the experiment. In class X, you will study a more formula definition of an event.
So, can you now tell what the events are in Activity 4?
With this background, let us now see what probability is. Based on what we directly observe as the outcomes of our trials, we find the experimental or empirical probability.

Let n be the total number of trials. The empirical probability P (E) of an event E happening, is given by

                   

In this chapter, we shall be finding the empirical probability, though we will write ‘probability’ for convenience,
Let us consider some examples.
To start with let us go back to Activity 2, and Table 15.2. in Column (4) of this table, what is the fraction that you calculate? Nothing, but it is the empirical probability of getting a head. Note that this probability kept changing depending on the number of trials and the number of heads obtained in these trials. Similarly, the empirical probability of getting a tail is obtained in Column (5) of Table. This is 12/15 to start with, then it is 2/3, then 28/45, and so on.
So, the empirical probability depends on the number of trials undertaken, and the number of times the outcomes you are looking for coming up in these trials.

Activity 

Before going further, look at the tables you drew up while doing Activity 3. Find the probabilities of getting a 3 when throwing a die a certain number of times. Also, show how it changes as the number of trials increases.  
Now, let us consider some other examples.