A sample is a subset of a population. Statistics, in general, is concerned with using samples to make inference about the parameters governing a larger (possibly infinite) population.

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13 views

Large difference in number of cases in each category of a variable

I want to run an analysis on a data set. However, the primary predictor variable (5 point scale) varies greatly in size of each group. I plan to create a dichotomous variable from these 5 category. ...
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1answer
14 views

Advice on which test to run to show a valid sample of participants

I have collected data on medical diagnosis and am wanting to know which test I should use to show my participants are similar to a pre and post intervention. The first set of data yielded 53% male ...
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22 views

Determine how much and why a sample differs from its population

I'd like to be able to determine a) how much and b) why a non-random sample “differs” from the population (there's probably a better word than “differs”; I was tempted to use “does not represent” but ...
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21 views

In search of a proper similarity function

I'm trying to find the most similar sample between a candidate and a bag of samples. Consider you have a knowledge corpus as follows: ...
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2answers
63 views

Alternative to z test for massive samples?

I'm trying to compare a number of large data sets (over 3000 samples per set), when I plot histograms of these distributions I can see two which are clearly different and a z-test confirms this, which ...
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2answers
41 views

How do you draw a random sample of unique IDs in a large dataset?

Rookie here -- I have a a large data set with about 75,000 observations, and 2000 unique IDs. Therefore, each unique ID has about 37 observations. I'm trying to draw a random sample of unique IDs, say ...
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2answers
192 views

Bootstrap: the issue of overfitting

Suppose one performs the so-called non-parametric bootstrap by drawing $B$ samples of size $n$ each from the original $n$ observations with replacement. I believe this procedure is equivalent to ...
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0answers
43 views

Should I use independent samples t-test?

I need to analyse if there's a difference between poor and rich European countries (using a GDP per capita index variable) and look if it has an effect on the variable environment. The GDP per capita ...
2
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1answer
85 views

Kolmogorov-Smirnov for Pareto distribution on sample

I want to use the Kolmogorov-Smirnov test to test if a sample is drawn from a Pareto distribution. Unfortunately, the only way to estimate the distribution's parameters is from the sample. Does ...
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1answer
32 views

Out-of-sample vs. test set

Someone asked me if I did out-of-time testing (which I assume is just out-of-sample testing but with a timeline element). But if I have a test set, is that not essentially the same as out-of-sample ...
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7 views

Select most different observations

I have a polar chart representing eight dimensional observations as 'rings' around the center. Furthermore, I am limited to Excel for graphing purposes, which means I can only display 255 series at ...
2
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39 views

Is a sample of 1961 people out of a population of 225,000,000 a sample size that is too small? If so, why?

I am working by a set of problems on a single question in my textbook. The question explained that there was a recent survey of 1961 adults out of a population of 225,000,000 and asked me to construct ...
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42 views

Out-of-time testing (basic question)

I understand the importance of out-of-sample testing, but could you tell me why I should (or shouldn't) do out-of-time testing ? The only use that comes to mind is if the predictive model applies to ...
3
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1answer
49 views

wilson's adjustment for sample proportion

Could somebody provide a derivation for Wilson's adjustment for the CI of a sample proportion? The formula is: $$\tilde{p} \approx \frac{x+2}{n+4}$$ where $x$ is the number of successes, and $n$ is ...
2
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1answer
32 views

Comparison of two means

Which test should I use, if I want to compare the mean of the whole sample with the mean of the fraction of the sample? So for example the mean of the whole classroom taking the test and the mean of ...
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1answer
9 views

procedure of statistical modeling of sample

I never did the statistical modeling of sample. Usually i was engaged in common statistical procedures(descriptive statistic, correlation, regression, factor analysis and so on). Now i decided find ...
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0answers
26 views

G*Power analysis -sample size calculation

My experiment is devised as follow: There are two different types of stimuli-- cellphones: android (android-1; android-2) and iOS (iOS-1 and iOS-2). A sample size of 60 subjects are divided into two ...
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1answer
52 views

What is the difference between sample and outcome? (plus events and observations)

I was sure that these are the same things but I do not get the difference reading about mass probability function Suppose that $X: S → A (A \subseteq R)$ is a discrete random variable defined on ...
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35 views

Stratified random sampling and its distribution

I'm learning stratified random sampling and I'm having hard time to solve this problem. consider the following population of six units: ...
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0answers
68 views

Binary logistic regression: low event rate

I want to investigate how people (lets say smokers and non-smokers) differ in various characteristics. So I want to perform a binary logistic regression with SPSS. However, my event rate is very low ...
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3answers
287 views

Why cannot a sample of voluntary patients be generalized?

In the book “OpenIntro Statistics” it is specified that samples of voluntary patients cannot be generalized. Why cannot a sample of voluntary patients be generalized?
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1answer
41 views

Is it true that values of parameters do not vary from sample to sample?

This is something I was told. However, it seems to me that parameters of a population can be functions of time, in certain situations, in which case the values of parameters could vary from sample ...
0
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1answer
65 views

$E(X_1| \overline X ) = \overline X$, the sample mean

Let $X_1, X_2, ..., X_n$ be a random iid sample from a population with mean $\theta$. Now I am wondering about the intuition behind $E(X_1| \overline X ) = \overline X$, the sample mean. If we just ...
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1answer
22 views

When can a pilot sample be included in the main study?

For example, when estimating the proportion of database records that agree with paper documentation. Auditing each record is tedious, so it is tempting to include the pilot sample in the main study ...
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0answers
21 views

Configural frequency analysis

What do you think about configural frequency analysis? http://en.m.wikipedia.org/wiki/Configural_frequency_analysis I personally love the idea to extract types and antitypes in heterogenous samples ...
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30 views

Choosing correct statistic aproach for dropout analysis

I need some input from you. At the moment, I feel a gap opening between theoretical knowledge and practical implementation. Problem case: In my study, I did an identic training program, around 10 ...
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1answer
80 views

Variance of sample correlation coefficient

On wiki page about Fisher transformation I read that variance of sample correlation coefficient becomes smaller as population correlation coefficient (in absolute value) gets closer to 1. Could ...
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1answer
19 views

Picking the top 10 answers from a survey

I ran a survey on a population of advisors in all the different recruitment centers of in a country. I had 1410 text written entries (not multiple choice) I took a sample of 284 to see what are the ...
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50 views

Weighted sample covariance

I have read the Wikipedia article, and know that the unbiased weighted sample covariance matrix for the row vector $\mathbf{x}_i$ is $$\Sigma=\frac{1}{\sum_{i=1}^{N}w_i - 1}\sum_{i=1}^N w_i ...
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27 views

Characterization of normal distribution [duplicate]

How can one prove that the random sample is normally distributed if the sample mean and sample variance are independent?
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48 views

Where can I download sample data sets for mobile app downloads?

For statistical testing, I am in need for android and apple mobile app download data. I have checked on AWS (Amazon) and Infochimps. No luck so far. Can anyone suggest other resources or is there a ...
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38 views

Random variables: True or False

I sometimes get confused when reading statistical definitions when they mention random variables (RV). Are they talking about a single draw? Are they talking about an estimator? To make things clear ...
2
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0answers
31 views

Difference between sample and sequence in Law of Large Numbers

I'm comparing the Chebychev (Weak) Law of Large Numbers (LLN) to the Kolmogorov (Strong) LLN in an econometric textbook and both definitions start off differently. The Chebychev LLN begins with If ...
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3answers
2k views

Is it a truly random sample?

Any hints on how to solve this problem? A sample of 400 items is taken from a normal population whose mean as well as variance is 4. If the sample mean is 4.5, can the sample be regarded as a ...
2
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0answers
34 views

Estimating the mean from knowing the first n largest values

There is a sample of n values that are the first n largest values of a population. Is there a way of getting any statistic such as mean or dispersion from such piece of information provided that the ...
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12 views

Mixed groups of participants

It is permissible to analyse data for a group of persons, which contains repeated measures data and independent measures data. Can I analyse this data as if it comes from one group? If not why not? ...
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1answer
58 views

Finding the level of significance of a sample

I have an exercise with the following information; $ 20\% $ of the set are youth, my sample of $ 139 $ persons have $ 39 $ youths. The exercise is to test if this sample is a representative sample of ...
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1answer
106 views

How to combine sample means and sample variances?

Given a set of data, assumed normally distributed from 1 to 19. And two subsets of that data, one with the numbers below 10, and one with the numbers above: ...
2
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1answer
40 views

Are samples from sliding windows independent samples?

Are samples from sliding windows independent samples? E.g. if I have window size of 90 seconds counting the number of cars on a street and I output the average within the window every second for 30 ...
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2answers
42 views

Check if sample is representative of a larger sample

From a larger sample of tabular data, I have picked certain rows that meet a certain condition (this condition is unrelated to the actual data in the rows). Now, I want to know if the distribution of ...
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0answers
16 views

Can I average samples in nested factor within-subject design if samples are from the same condition?

This question relates to one of my other questions: Split plot with nested factors ANOVA in R . I have subjects taking an experiment, where I have different ...
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2answers
134 views

Estimating the size of an intersection of multiple sets by using a sample of one set

I'm working on an algorithm which needs to calculate the size of a set generated by the intersections of at least 2 sets. More specifically: $$ z = \left |A_0 \cap \ldots \cap A_n \right | $$ The ...
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1answer
70 views

bias and variance of correlation estimator

I calculated the bias and variance of sample mean $\hat{\mu}$ and sample variance $\hat{\sigma}^2$ but I could not calculate the bias and variance for sample correlation $\hat{\rho}$. How can I do the ...
2
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1answer
41 views

Proportionality of significance and effect size

Is it true that, given a fixed sample size N, it can be said that some measure of effect size and the measure of statitical significance (e.g., 1-p) are proportional? In other words, that, when ...
0
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1answer
49 views

NBA Points Per Game

I have a database with many thousands of entries of box scores. Each box score has a player name, date played, points scored, minutes played, and some other stats like rebounds, assists, etc. I want ...
3
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1answer
48 views

Problems with the sample (unkown population)

I realized a large research with Brazilian physics high-school teachers and I wanted that my sample could be representative of the Brazilian physics high-school teachers population. However, nobody in ...
0
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1answer
117 views

Combining results of two samples to find combined standard deviation

I was making an attempt to solve the following problem sets, however, my answer 15.913 seemed quite different from that in the answer key 16.84. I have checked my answer a few times and re-doing the ...
0
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1answer
48 views

Can Inferential Statistics be carried out in a Retrospective Case Study?

Just wanted to check with regard to a retrospective case study, if various data is collected for a set of patients on different variables can this be interpreted with inferential stats tests? Even if ...
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0answers
14 views

How can I determine the probability that a sample has a certain average given details about the total population?

Imagine that I have goal scoring data for a hockey team. They average 2.43 goals scored per game with a standard deviation of 1.63 for this data set. They have played 82 games. I have two subsets of ...
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19 views

Computationally cheap estimation of the mode from 2D samples

I say 'mode' in the title – perhaps there's something more appropriate I can find. Here's my problem: A person is in a $100\mathrm m$ by $500\mathrm m$ space. I have about 100 samples randomly ...