# Without data, all you are is just another person with an opinion.

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1 OCR Statistics Module Revision Sheet The S exam is hour 30 minutes long. You are allowed a graphics calculator. Before you go into the exam make sureyou are fully aware of the contents of theformula booklet you receive. Also be sure not to panic; it is not uncommon to get stuck on a question I ve been there!). Just continue with what you can do and return at the end to the questions) you have found hard. If you have time check all your work, especially the first question you attempted...always an area prone to error. Without data, all you are is just another person with an opinion. Representation Of Data J.M.S. You must be happy constructing unordered, back-to-back and ordered stem and leaf diagrams. They show the overall distribution of the data and back-to-back diagrams allow you to compare two sets of data. Cumulative frequency graphs. The cumulative frequency is a running total of the frequencies as you go up the values. For example x f x upper limit of) cum. freq 0 x < 5 8 Create x < 0 3 Cumulative 0 0 x < 5 7 Frequency x < Plot the second of these tables and join it with a smooth curve to form the cumulative frequency curve. From this the median and the two quartiles can be found. Once these values are found we can draw a box and whisker diagram. The box and whisker diagram uses five values: the minimum, the maximum, the lower quartile, the upper quartile and the median. It is good for showing spread and comparing two quantities. min. LQ med. UQ max. Histograms are usually drawn for continuous data in classes. If the classes have equal widths, then you merely plot amount against frequency. If the classes do not have equal widths then we need to create a new column for frequency density. Frequency density is defined by f.d. = frequency class width. The area of the bars are what represents the frequency, not the height. Frequency polygons are made by joining together the mid-points of the bars of a histogram with a ruler. J.M.Stone

2 Measures Of Location The mean arithmetic mean) of a set of data {x,x,x 3...x n } is given by sum of all values x x = the number of values = n. When finding the mean of a frequency distribution the mean is given by xf) xf) =. f n If a set of numbers is arranged in ascending or descending) order the median is the number which lies half way along the series. It is the number that lies at the ) n th position. Thus the median of {3,4,5,5} lies at the position average of 4 and 5 median = 4.5. The mode of a set of numbers is the number which occurs the most frequently. Sometimes no mode exists; for example with the set {,4,7,8,9,}. The set {,3,3,3,4,5,,,,7} has two modes 3 and because each occurs three times. One mode unimodal. Two modes bimodal. More than two modes multimodal. Advantages Disadvantages Mean The best known average. Greatly affected by extreme values. Can be calculated exactly. Can t be obtained graphically. Makes use of all the data. When the data are discrete can give an impossible figure.34 children). Can be used in further statistical work. Median Can represent an actual value in the For grouped distributions its value can data. Can be obtained even if some of the values in a distribution are unknown. only be estimated from an ogive. When only a few items available or when distribution is irregular the median may not be characteristic of the group. Unaffected by irregular class widths and unaffected by open-ended classes. Can t be used in further statistical calculations. Not influenced by extreme values. Mode Unaffected by extreme values. May exist more than one mode. Easy to calculate. Can t be used for further statistical work. Easy to obtain from a histogram. When the data are grouped its value cannot be determined exactly. Measures Of Spread The simplest measure of spread is the range. Range = x max x min. The interquartile range is simply the upper quartile take away the lower quartile. Both of these values are usually found from a cumulative frequency graph above). The sum of squares from the mean is called the sum of squares and is denoted S xx = x x) = x nx. Statistics argues that the average person has one testicle and that % of people have more than the average number of arms... J.M.Stone

3 For example given the data set {3,,7,8} the mean is ; x = = 58; so S xx = x nx = 58 4 = 4. The standard deviation σ) is defined: σ = variance = Sxx x n = n x. Example: Given the set of data {5, 7, 8, 9, 0, 0, 4} calculate the standard deviation. Firstly we note that x = 9. x 5 σ = n x = 4 ) = 8 = x When dealing with frequency distributions such as, we could calculate σ by writing out the data 3 and carrying out the calculations as above, but this f is clearly slow and inefficient. To our rescue comes a formula for σ that allows direct calculation from the table. This is x σ = f) x. n Example: Calculate mean and sd for the above frequency distribution. For easy calculation we need to add certain columns to the usual x and f columns thus; xf) So x = n x f xf x f n = f = 5 xf) = 75 x f) = 7. = 75 x 5 = 3 and σ = f) 7 x = n 5 3 =.9... Linear Coding. Given the set of data {,3,4,5,} we can see that x = 4 and it can be calculated that σ =.44 3dp). If we add 0 to all the data points we can see that the mean becomes 4 and the standard deviation will be unchanged. If the data set is multiplied by 3 we can see that the mean becomes and the standard deviation would become three times as large dp)). If, instead of being given x and x, you were given x a) and x a) for some constant a, you just use the substitution u = x a and use u and u to work out the mean of u and the standard deviation of u. Then, using the above paragraph, we know x = ūa and σ x = σ u. Or we could have done S xx = Σx x) = 3 ) ) 7 ) 8 ) = 4. 3 {,,,,,,,,,3,3,3,3,3,3,3,4,4,4,4,4,5,5,5,5}!!! 3 J.M.Stone

4 Probability An independent event is one which has no effect on subsequent events. The events of spinning a coin and then cutting a pack of cards are independent because the way in which the coin lands has no effect on the cut. For two independent events A & B PA and B) = PA) PB). For example a fair coin is tossed and a card is then drawn from a pack of 5 playing cards. Find the probability that a head and an ace will result. Phead) =, Pace) = 4 5 = 3, so Phead and ace) = 3 =. Mutually Exclusive Events. Two events which cannot occur at the same time are called mutually exclusive. The events of throwing a 3 or a 4 in a single roll of a fair die are mutually exclusive. For any two mutually exclusive events PA or B) = PA)PB). For example a fair die with faces of to is rolled once. What is the probability of obtaining either a 5 or a? P5) =, P) =, so P5 or ) = = 3. Non-Mutually Exclusive Events. When two events can both happen they are called nonmutually exclusive events. For example studying English and studying Maths at A Level are non-mutually exclusive. By considering a Venn diagram of two events A & B we find PA or B) = PA)PB) PA and B), PA B) = PA)PB) PA B). Tree Diagrams. These may be used to help solve probability problems when more than one event is being considered. The probabilities on any branch section must sum to one. You multiply along the branches to discover the probability of that branch occurring. For example a box contains 4 black and red pens. A pen is drawn from the box and it is not replaced. A second pen is then drawn. Find the probability of i) two red pens being obtained. ii) two black pens being obtained. iii) one pen of each colour being obtained. iv) two red pens given that they are the same colour. Draw tree diagram to discover: R = 5/9 R = /0 B = 4/0 B = 4/9 R = /9 i) Ptwo red pens) = = = 3. ii) Ptwo black pens) = = 90 = 5. iii) Pone of each colour) = = 8 5. iv) Ptwo reds same colour) = /3 /3/5 = 5 7. B = 3/9 4 J.M.Stone

5 Conditional Probability. In the above example we see that the probability of two red pens is 3, but the probability of two red pens given that both pens are the same colour is 5 7. This is known as conditional probability. PA B) mean the probability of A given that B has happened. It is governed by PA B) = PA and B) PB) = PA B). PB) For example if there are 0 students in a year and 0 study Maths, 40 study English and 0 study both then Pstudy English study Maths) = Pstudy Maths & English) Pstudy Maths) = 0/0 0/0 =. A is independent of B if PA) = PA B) = PA B ). i.e. whatever happens in B the probability of A remains unchanged.) For example flicking a coin and then cutting a deck of cards to try and find an ace are independent because Pcutting ace) = Pcutting ace flick head) = Pcutting ace flick tail) = 3. Permutations And Combinations Factorials are defined n! n n ) n ). Many expressions involving n! factorials simplify with a bit of thought. For example n )! = nn ). Also there is a convention that 0! =. The number of ways of arranging n different objects in a line is n! For example how many different arrangements are there if 4 different books are to be placed on a bookshelf? There are 4 ways in which to select the first book, 3 ways in which to choose the second book, ways to pick the third book and way left for the final book. The total number of different ways is 4 3 = 4! Permutations. The number of ways of selecting r objects from n when the order of the selection matters is n P r. It can be calculated by n P r = n! n r)!. For example in how many ways can the gold, silver and bronze medals be awarded in a race of ten people? The order in which the medals are awarded matters, so the number of ways is given by 0 P 3 = 70. In another example how many words of four letters can be made from the word CON- SIDER? This is an arrangement of four out of eight different objects where the order matters so there are 8 P 4 = 8! 4! = 80 different words. Combinations. The number of ways of selecting r objects from n when the order of the selection does not matter is n C r. It can be calculated by ) n n n! C r = r r!n r)!. For example in how many ways can a committee of 5 people be chosen from 8 applicants? Solution is given by 8 C 5 = 8! 5! 3! = 5. In another example how many ways are there of selecting your lottery numbers where one selects numbers from 49)? It does not matter which order you choose your numbers, so there are 49 C = possible selections. 5 J.M.Stone

6 If letters are repeated in a word, then you just divide through by the factorials of each repeat. Therefore there are! 4! 4!! arrangements of the word MISSISSIPPI. 4 You must be good at choosing committee questions [be on the lookout, they can be in disguise]. For example how many ways are there of choosing a committee of 3 women and 4 men from a group containing 0 women and 5 men? There are 0 3) ways of choosing the women the order doesn t matter) and 5 4) ways of choosing the men. Therefore overall there are 5 4) 0 3) ways of choosing the committee. Example: If I deal six cards from a standard deck of cards, in how many ways can I get exactly four clubs? Well there are 3 4) ways of getting the clubs, and 39 ) ways of getting the non-clubs, so therefore the answer to the original question is 3 4) 39 ). When considering lining things up in a line we start from the principle that there are n! ways of arranging n objects. In the harder examples you need to be a cunning. For example three siblings join a queue with 5 other people making 8 in total.. How many way are there of arranging the 8 in a queue? Easy; 8!. How many ways are there of arranging the 8, such that the siblings are together? We, we imaging the three siblings tied together. There are therefore! ways of arranging the 5 and the bundle of siblings and then there are 3! ways of arranging the siblings in the bundle. Therefore the answer is! 3! 3. How many ways are there of arranging the siblings so they are not together? There are 5! ways of arranging the five without the siblings. There are then places for the first sibling to go, 5 for the second, and 4 for the third. Therefore 5! 5 4. To calculate probabilities we go back to first principles and remember that probability is calculated from the number of ways of getting what we want over the total number of possible outcomes. So in the above example, if the 8 are arranged randomly in a line, what is the probability of the siblings being together? Ptogether) =! 3! 8!. Going back to the four club question if it asked for the probability of getting exactly four clubs if I dealt exactly six cards from the pack, the answer would be 3 4) 39 ). The 5) 5 ) represents the total number of ways I can deal six cards from the 5. Probability Distributions A random variable is a quantity whose value depends on chance. The outcome of a random variable is usually denoted by a capital letter e.g. X). We read PX = ) as the probability that the random variable takes the value. For a fair die, PX = 5) =. For discrete random variables they are usually presented in a table. For example for a fair die: x PX = x) In general, for any event, the probability distribution is of the form x x x x 3 x 4 x 5 x PX = x) p p p 3 p 4 p 5 p 4 As an non-mathematical aside, find two fruits that are anagrams of each other. J.M.Stone

7 As before, it is crucial that we remember the probabilities sum to one. This can be useful at the start or problems where a constant must be evaluated. For example in: We discover kkk 4k =, so k = 8. x 3 4 PX = x) k k k 4k Binomial And Geometric Distributions The binomial distribution is applicable when you have fixed number of repeated, independent trials such that each trial can be viewed as success p) or fail q = p). In justifying a binomial distribution you must not just quote the previous sentence; you must apply it to the situation in the question. For example: Binomial is applicable because the probability of each tulip flowering is independent of each other tulip and the probability of flowering is a constant. For example if I throw darts at a dart board and my chance of hitting a double is 0. and I throw darts at the board and my chance of hitting a double is independent of all the other throws then a binomial distribution will be applicable. We let X be the number of doubles I hit. X can therefore take the values {0,,...,,}; i.e. there are 3 possible outcomes. p = 0.; the probability of success and q = p = 0.9; the probability of failure. We write X Bn,p) which here is X B,0.). I would always advise you to visualise the tree diagram. From this we can see that PX = ) = 0. and PX = 0) = 0.9. In general ) n PX = x) = p x q n x. x So in the example, the probability I hit exactly 7 doubles is PX = 7) = 7) For questions such as PX 5) or PX 8) you must be able to use the tables in the formula book. The tables always give PX something). You must be able to convert probabilities to this form and then read off from the table. For X B0,0.35). PX 7) = 0.995, PX < 5) = PX 4) = 0.755, PX 7) = PX ) = = 0.00, PX > 3) = PX 3) = = The geometric distribution is applicable when you are looking for how long you wait until an event has occurred. The events must be repeated, independent and success/fail. Potentially you could wait forever until a success occurs; something to look for if you are unsure what distribution to apply. Similar to the binomial you must justify in the context of the question. Going back to the darts example, we could rephrase it as how long must I wait until I hit a double? Let X be the number of throws until I hit a double. We write X Geo0.). X can take the values {,,3,...}. Obviously PX = ) = 0.. Less obviously PX = 4) = I must have three failures and then my success). In general PX = x) = q x p. 7 J.M.Stone

8 There are no tables for the geometric distribution because there does not need to be. To calculate PX 5) wemusthave had4failures. ThereforePX 5) = q 4 = p) 4. Also tocalculate PX ) weusethefactthat PX ) = PX 7) = q = p). In general PX x) = p) x and PX x) = p) x. Expectation And Variance Of A Random Variable The expected value of the event is denoted EX) or µ. It is defined For example for a fair die with EX) = µ = xpx = x). x PX = x) we find: EX) = ) = 3. ) 3 ) 4 ) 5 ) ) The variance of an event is denoted VarX) or σ and is defined VarX) = σ = EX ) EX)) = EX ) µ = x PX = x) µ. So for the biased die with distribution we find that and EX) = 3) VarX) = x PX = x) µ x PX = x) ) 3 0)4 0) 5 ) ) 3 = 3 = 3) ) 3 0 ) 4 0 ) 5 ) 3) 3 = 7 3 = 4. The expectation of a binomial distribution Bn,p) is np. The variance of Bn,p) is npq. The expectation of a geometric distribution Geop) is p. 8 J.M.Stone

9 Correlation The Product Moment Correlation Coefficient is a number r) calculated on a set of bivariate data that tells us how correlated two data sets are. The value of r is such that < r <. If r = you have perfect positive linear correlation. If r = you have perfect negative linear correlation. If r = 0 then there exists no correlation between the data sets. It is defined r = xi x)y i ȳ) xi x) y i ȳ) = S xy Sxx S yy where we define the individual components as So to calculate r for the data set S xx = x n x), S yy = y n y), S xy = xy n x y. x y we write the data in columns and add extra ones. We then sum the columns and calculate from these sums. Note that in the above example n = 8 i.e. the number of pairs, not the number of individual data pieces). Therefore x y x y xy S xx = x n x) = = 59.5, S yy = y n y) = = , S xy = xy n x y = = Therefore r = S xy Sxx S yy = = Therefore the data has very, very weak negative correlation. Basically it has no meaningful correlation. It can be shown that if one or both) of the variables are transformed in a linear fashion i.e. if we replace the x values by, say, x 4 3 or any transformation formed by,, or with constants) then the value of r will be unchanged. 9 J.M.Stone

10 You need to be able to calculate Spearman s rank correlation coefficient r s ). You will be given a table and you will need to in the next columns) rank the data. If two data points are tied then you e.g. the nd and 3rd are tied) then you rank them both.5. % IQ Rank % Rank IQ d d Now r s = d nn ). d is just the sum of the d column in the table and n is the number of pairs of data; here n = 9. We therefore find r s =.5 98 ) = Therefore we see a strong degree of positive association. If r s is close to then strong negative association. If close to zero then no meaningful association/agreement. Regression For any set of bivariate data x i,y i ) there exist two possible regression lines; y on x and x on y. If neither is controlled see below) then if you want to predict y from a given value of x, you use the y on x line. If you want to predict x from a given value of y, you use the x on y line. The y on x line is defined y = abx where b = S xy S xx and a = ȳ b x. The x on y line is defined x = a b y where b = S xy S yy and a = x b ȳ. Both regression lines pass through the average point x, ȳ). In the example in the book P80) the height of the tree is the dependent variable and the circumference of the tree is the independent variable. This is because the experiment has been constructed to see how the height of the tree depends on its circumference. If one variable is being controlled by the experimenter e.g. x), it is called a controlled variable. If x is controlled you would never use the x on y regression line. Only use the y on x line. You would use this to predict y from x expected) and x from y not-expected) 0 J.M.Stone

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