Introduction to standard deviation

Standard deviation

The most comprehensive descriptions of dispersion are those that deal with the average deviation from some measure of central tendency. Two of these measures are important to our study of statistics: the variance and the standard deviation. Both of these tell us an average distance of any observation in the data set from the mean of the distribution.

Standard deviation definition
Earlier, when we calculated the range, the answers were expressed in the same units as the data. For the variance, however, the units are the squares of the units of the data – for example, “squared dollars” or “dollars squared”. Squared dollars or dollars squared are not intuitively clear or easily interpreted. For this reason, we have to make a significant change in the variance to compute useful measure of deviation one that does not give us a problem with units of measure and thus is less confusing. According to the , definition of standard deviation, this measure is called the standard deviation, and it is the square root of the variance. The square root of 100 dollars squared is 10 dollars because we take the square root of both the value and the units in which it is measured. The standard deviation, then, is in units that are the same as the original data.

Let us now define standard deviation: The standard deviation is a numerical measure of the average variability of a data set around its mean. The standard deviation for a population is denoted by s (Greek lower case letter sigma) and standard deviation for a sample denoted by s.The mean deviation has a limitation that it ignores the sign of x -   in the general case of an observation x. The standard deviation gets over this limitation by squaring x -  . (x -  )2 is positive whether or not x -   is negative or positive. Note that population means data set comprising of 100% items under study, and sample means data set comprising of sample items drawn out of the population so that by studying the sample, inferences may be drawn about the population.

Sequences and Series- An introduction

Sequence:


Let us consider the following collection of numbers – 
(1) 28,2,25,27,--------
(2) 2,7,11,19,31,51, ---------
(3) 1,2,3,4,5,6, --------------
(4) 20.5,18.5,16.5,14.5,12.5,10.5, ------------
In (1) the numbers are not arranged in a particular order. In (2) the numbers are in ascending order but they do not obey any rule or law. It is, therefore not possible to indicate the number next to 51.
In (3) we find that by adding 1 to any number, we get the next number. So the number after 6 would be = 6+1 = 7.
In (4) if we subtract 2 from any number we get the number that follows. So here the number after 10.5 would be = 10.5-2 = 8.5
Under these circumstances, we say, the numbers in the collections (1) and (2) do not form sequences whereas the numbers in the collections (3) and (4) form sequences.
Definition: An ordered collection of numbers a1,a2,a3,a4,….. an,…… is called a sequence if according to some definite rule or law there is a definite value of an, called the term or element of the sequence, corresponding to any value of the natural number n.
Clearly the nth term of a sequence is a function of the positive integer n. If the nth term itself is also always an integer, then such a sequence is called an integer sequence.

Series:

An expression of the form a1+a2+a3+a4+….. +an+….. which is the sum of the elements of a sequence {an} is called a series.

If Sn = u1+u2+u3+….un, the Sn is called the sum to n terms (or the sum of first n terms) of the series and is denoted by the Greek letter sign ?.
Thus Sn = ?_(r=1)^n¦u_r

If a sequence or a series contains finite number of elements, it is called a finite sequence or series, otherwise they are called infinite sequences and series.

Progressions:

There are mainly three types of progressions –
(a) Arithmetic progression (A.P.): That means a sequence in which each term is obtained by adding a constant d to the preceding term. This constant ‘d’ is called the common difference of the arithmetic progression.
(b) Geometric progression (G.P.): If in a sequence of terms each term is constant multiple of the proceeding term, then the sequence is called a Geometric Progression (G.P.). The constant  multiplier is called the common ratio (r).
(c) Harmonic progression (H.P.): If each term of an A.P. is replaced by its reciprocal, then we get a harmonic progression.

type of polygons

Few names of polygons are as follows:

• Triangle or Trigon

This is a three-sided shape that having whole interior angle measurement as 180º.

• Quadrilateral or Tetragon

This is a four-sided shape that having whole inside angle measurement as 360º.

• Pentagon

This is a five-sided shape which contains the whole inside angle measurement as 540º.
The polygon is a closed path. The polygon has the different shapes. The shapes are depends on the number of sides. The Straight lines are form the polygon shapes. The polygons are having more number of straight lines to form the different polygons. Angles of the polygons are varied based on shape of the polygon.
The area of the regular polygon can be written as,
Area = S² N / 4 tan (pi / N).
Here, S = Length of any side, N = Number of side, Pi = 3.14.

Area of the Polygons

1) The given length of side:
Area Shape of polygon = S² N / 4 tan (Pi/N)
Here, S = Side length, N = Number of side, Pi = 3.14.
2) Given the radius(circum radius):
Area shape of polygon = (R² N sin (2Pi/N))/2.
Here, R = Radius of the polygon, N = Number of side, Pi = 3.14.
3) Given the apothem (In radius):
Area Shape of polygon = A² N tan (Pi/N).
Here, A = Apothem length, N = Number of sides.
4) Given the apothem and Length of a side:
Area shape of polygon = A* P / 2.
Here, A = Apothem length, P = Perimeter.

More about Polygon:

Basic diagrammatic representation for polygon is as follows:

Polygon- A polygon is a closed figure made by joining line segments, where each line segment intersects exactly two others.

Regular Polygon- A regular polygon is a polygon whose sides are all the same length, and whose angles are all the same.

Quadrilateral- A four-sided polygon. The sum of the angles of a quadrilateral is 360^o.

Rectangle- A four-sided polygon having all right angles. The sum of the angles of a rectangle is 360^o.

Square- A four-sided polygon having equal-length sides meeting at right angles. The sum of the angles of a square is 360^o.

Parallelogram- A four-sided polygon with two pairs of parallel sides. The sum of the angles of a parallelogram is 360^o.

Rhombus- A four-sided polygon having all four sides of equal length. The sum of the angles of a rhombus is 360^o.

Trapezoid- A four-sided polygon having exactly one pair of parallel sides. The two sides that are parallel are called the bases of the trapezoid. The sum of the angles of a trapezoid is 360^o.

Pentagon- A five-sided polygon. The sum of the angles of a pentagon is 540^o.

Hexagon- A six-sided polygon. The sum of the angles of a hexagon is 720^o.

Octagon- An eight-sided polygon. The sum of the angles of an octagon is 1080^o.

Nonagon- A nine-sided polygon. The sum of the angles of a nonagon is 1260^o.

Decagon- A ten-sided polygon. The sum of the angles of a decagon is 1440^o.

Convex- A figure is convex if every line segment drawn between any two points inside the figure lies entirely inside the figure. A figure that is not convex is called a concave figure.

Area of a square = side x side

= s x s

= s² sq units or units²

Area of a Rectangle = length x width

= l x w

= lw sq units or units²

Area of a Parallelogram= base x height

= b x h

= bh sq units or units²

Area of a Triangle = ½ x base x height

= ½ b h sq units or units²

Area of a Rhombus = base x height

= b x h

= bh sq units or unit²

Area of a Trapezoid = ½ (a+b)h sq units or units²

(Half of the sum of the lengths of the

Parallel sides times height)

Statistics: Bias and Sample Bias

In statistics we come across two types of errors, random errors and systematic errors. Random error is the error due to sampling variability or at times measurement precision. It occurs essentially in all quantitative studies and can be minimized to an extent but not avoided. We might wonder, What is Bias? Bias or Systematic error is a reproducible inaccuracy that produces a consistently false pattern of differences between observed (estimated) and true values. We get Definition Bias or definition of Bias as the systematic or average difference between the true value of the parameter and the experimental estimate value.

Bias Statistics or Statistical Bias is an error we cannot correct by repeating an experiment many times and averaging together the results, it is a directional error in an estimator.In a population which includes 50% males and 50% females, if we know that an important variable is the gender. Then the sample needs to include the same proportion. But if the sample includes 30% males then we can say that the results are likely biased as there are not enough responses from the male gender. This is how we know Sample Bias or in other words if the given sample is biased.

In statistics, Bias is used to refer the directional error in an estimator. Though we repeat the same experiment many times there would be some randomness existing.  By repeating an experiment we might see a slight variation in the estimation values. The bias is a systematic or average difference between the experimental estimate and the true value.

Examples of Bias
For example, if the national census could be completed online using the internet, then the sampling is said to be biased as only those people who could afford a computer and internet connection would be included and the people who cannot afford a computer or have the necessary skills to operate one will be excluded.

Jason was assigned by his editor to determine what most Americans think about the new law that will place a special tax on all electronic goods purchased and was told to mention in the survey form that he revenue returns collected from the tax would be made use of to enforce new online decency laws. Jason decided to use the email poll for convenience. In this poll 90% of those surveyed opposed the tax. Jason was quite surprised when 65% of all Americans voted for the taxes. So, online and call-in polls are particularly biased as the respondents are self-selected.

For example, a movie is released and a survey is done on the first day first show to know how people liked it. In the survey some say the movie was awesome and they all liked it and some say the movie was boring and they did not like it at all. Here ‘they’ might refer to only their group.

Inverse Function

What is an Inverse Function?

Inverse Function

In real life, did you ever undo the activity you or some other person has performed? If yes, then you have applied inverse function in your life. The inverse function is a function which will undo another function.  For example, let us consider that you have to call your friend using your mobile phone. When you wanted to dial to your friend, you will open the phone book, select his name and click the call button. What happens internally? The phone number of the corresponding name is retrieved and a call is established to that number. In the inverse way, what happens when your friend calls you? You get his name listed in the caller id. This is because; your friend’s phone number reaches your phone. The number gets converted into name and gets displayed. Thus, while calling your friend, the name gets converted into number. While receiving the call, the inverse of function happens and the number gets converted into the name.

However, the inverse of a function must result in the function itself i.e. assume that John is in your contact list and his number is 9904567345. If you call John from your phone book, then the call goes to 9904567345. In that case, when you receive a call back from 9904567345, it should be displayed as John in your phone and not some other name.

Here is another example for you: If a function represents the statement: “John is the father of Dave” then the inverse function will be: “Dave is the son of John”.

How to find Inverse Function?
Now that we know what the inverse of a function is, let us concentrate on how to find inverse function. Finding the inverse of a function can be demonstrated using the algebraic equation x = 4y + 3.

For finding inverse functions of x=4y+3, follow the steps given below:

Step 1: Move the + 3 on right side to the left side. Inverse of addition is subtraction. Thus when the +3 goes to the left side, it becomes -3. Thus the function will be evolved into x-3 = 4y.

Step 2: Now the number 4 has to be moved from right side to left side. Number 4 which is multiplied with y has to move to left side. Inverse of multiplication is division. Thus the 4 on the right hand side will perform division when moving to the left side. The function will now be transformed into (x-3)/4 = y.

Thus the inverse function of x=4y+3 is y=(x-3)/4.

Learn Exponential functions

Definition of Exponential functions: Exponential functions  are those function in which independent variable appear as an exponent .
exponential functions examples : y = 3x , f(x) = ex , y = 32x+5  are an example of exponential function.
Exponential function
Y = ax, where a is a positive real number and a ?0  is the most popular exponential function . It is an increasing function (as the value of x increasing the value of y increases ) when a>1, It is a decreasing function (as the value of x decreases the value of y decreases ) when a<1.
Derivative of exponential function :
We will use the following property to derivate  exponential function such as
  (ex) = ex

Least common multiple and Least common denominator

Least common multiple or LCM of two numbers is the smallest number that is multiple of both.
How to find the least common multiple?
Methods to find the LCM:
1. Prime factorization method – Following steps are followed to find LCM by prime factorization method:
• Find the factors of numbers whose LCM has to be obtained.
• Find the common factors of all the numbers.
• Find the factors that are not common.
• LCM is the product of common factors and the factors that are not common
LCM = (Product of common factors) X (Product of uncommon factors)
2. Common division method – Following steps are followed to find LCM by common division method: -
• Arrange numbers together separated by commas.
• Start with smallest prime number and keep on dividing till none of the numbers can further be divided.
• Multiply all the factors together to find the LCM.
The common division method is useful for finding the LCM of more than two numbers.
Least common denominator
Least common denominator is calculated for adding and subtracting the fractions. We cannot add or subtract any fraction before finding the least common denominator.
 How to find the least common denominator? (Finding the least common denominator)
Least common denominator finder
Suppose if we have two fractions such as 7/4 and 5/6.
To add these fractions, we need to find the least common denominator and o find the least common denominator of these two fractions, we find the least common multiple of their denominators i.e. LCM of 4 and 6.
4 = 2 X 2
6 = 2 X 3
2 is the only common factor and 2, 3 are the factors that are not common.
Product of common factors = 2
Product of factors that are not common = 2 X 3 = 6
LCM = (Product of common factors) X (Product of factors that are not common)
= (2) X (2 X 3) = 2 X 6 = 12
Hence the LCM of 4 and 6 is 12.
The least common denominator of 7/4 and 5/6 is 12
7/4 + 5/6 = 7*3/4*3 + 5*2/6*2 = 21/12 + 10/12 = 31/12