Group PresentationSampling Methods Project

STA550 Group Members:

AMIRA BINTI MISDAR 2012738515HATIKA BINTI MEGAT JAMIL 2012959065NUR ARINA BINTI NURUL ‘AKLA 2012918743NUR FATHANAH BINTI MOHAMAD ZAKI2012753899

Sept 2013 – Jan 2014D2 CS 241 4A

QUESTION 1

DISTINGUISH CAREFULLY BETWEEN A PILOT SURVEY, A SAMPLE

SURVEY AND A CENSUS.

Pilot Survey

A small experiment designed to test logistics and gather information prior to a larger study, in order to improve the latter’s quality and efficiency.

It also can reveal deficiencies in the design of a proposed experiment or procedure and these can be addressed before time and resources are expended on large scale studies.

Pilot Survey cont…

To test out the questions and see if they are giving you the type of answers that you want.

People may not understand what you are asking, so the reseacher may need to modify questions to get what you want. If you do not carry a pilot test and not work out on those problems, the results from the final questionnaire may be useless.

Reasons :

SAMPLE SURVEYUse to obtain information about a large aggregate or population by selecting and measuring a sample from that population.

Due to the variability of characteristics among items in the population, researchers apply scientific sample designs in the sample selection process to reduce the risk of a distorted view of the population,

And then, they make inferences about the population based on the information from the sample survey data.

SAMPLE SURVEY CONT…

• A sample survey costs less than a census because data are collected from only part of a group.COST

• Results are obtained far more quickly for a sample survey, than for a census. Fewer units are contacted and less data needs to be processed.

TIME

• The smaller scale of this operation allows for better monitoring and quality control.

CONTROL

• Fewer people have to respond in the sample.RESPONSE

BURDEN

REASONS

• A census is a survey conducted on the full set of observation objects belonging to a given population or universe.

• The United Nations defines the essential features of population and housing censuses as "individual enumeration, universality within a defined territory, simultaneity and defined periodicity", and recommends that population censuses be taken at least every 10 years.

Census

Census cont…

Census data are

published in a wide variety of formats to

be accessible

.

Data can be represented visually or analyzed in

complex statistical models.

Census data offer a unique

insight into small areas and small

demographic groups.

Reasons:

QUESTION 2

WHY RESULTS FROM A CENSUS MIGHT DIFFER

FROM THE TRUE VALUES IN THE POPULATION?

ERROR IN STATISTICAL DATA

The accuracy of a survey estimate refers to the closeness of the estimate to the true population value. Where there is a discrepancy between the value of the survey estimate and true population value, the difference between the two is referred to as the error of the survey estimate.

WHAT IS

ERROR?

Error (statistical error) 

describes the difference between a value

obtained from a data collection process and the 'true' value for the

population. 

ERROR??ERROR???

ERROR???

SAMPLING ERRORSampling error which

arises when only a part of the population is used to represent

the whole population.

It occurs solely as a result of using

a sample from a population, rather than conducting a

census the population.

It refers to the difference between an estimate for a population based on data

from a sample and the 'true' value for that population

which would result if a census were taken.

SAMPLING ERROR cont…

POPULATION SPECIFICATION ERROR

• This error occurs when the researchers does not understand who they should survey.

SAMPLE FRAME ERROR

• A frame error occurs when the wrong sub-population is used to select a sample.

SELECTION ERROR • This occurs when respondents self select their participation in the study – only those that are interested respond. Selection error can be controlled by going extra lengths to get participation.

TYPES

Sampling Error cont…

Careful sample designs

Large samples

Multiple contacts to assure represent

ative response.

• Caused by factors other than those related to sample selection.

• Occur at any stage of a sample survey and can also occur with censuses.

• It refers to the presence of any factor, whether systemic or random.

• Results in the data values not accurately reflecting the 'true' value for the population. 

NON-SAMPLING ERROR

• Unit in the sample is incorrectly excluded or included, or is duplicated in the sample.

• The failure to obtain a response from some unit

• because of absence, non-contact, refusal, or some other reason.

• Respondents intentionally or accidentally providing inaccurate responses.

• Concepts, questions or instructions are not clearly understood by the respondent.

NON-SAMPLING ERROR cont...

INTERVIEWER ERROR

• Interviewers incorrectly record information; are not neutral or objective; influence the respondent to answer in a particular way; or assume responses based on appearance or other characteristics.

PROCESSING ERROR

• Errors that occur in the process of data collection, data entry, coding, editing and output.

NON-SAMPLING ERROR cont...

WHY ERROR DOES MATTER?

The greater the errors, the less reliable are the results of the study.

A credible data source will have measures in place throughout the data collection process to minimize the amount of error.be transparent about the size of the expected error so that users can decide whether the data are 'fit for purpose'.

HOW DO WE MEASURE ERROR?

Two common measures of error:Standard Error (SE)

- measure of the variation between any estimated population

value - based on a sample

rather than true value for the population. 

Relative Standard Error (RSE)

- standard error expressed as a

proportion of an estimated value.

- useful measure as they provide an indication of the

relative size of the error likely to have

occurred due to sampling.

WHAT CAN MEASURES OF ERROR TELL US?

The standard error can be used to construct a confidence interval. 

• C.I is a range in which it is estimated the true population value lies.

• C.I of different sizes can be created to represent different levels of confidence that the true population value will lie within a particular range.

REDUCING ERRORS

IN SURVEY

WAY TO REDUCE ERROR MEASUREMENT

CALLBACKS REWARD AND INCENTIVES

TRAINED INTERVIEWERS

DATA CHECKS QUESTIONNAIRES CONSTRUCTION

Example:

We want to conduct a survey about the UiTM cafeteria in UiTM Machang. The survey is based on the customers’ satisfaction on the food, price, facilities and services provided by every cafeteria in UiTM Machang. The customers’ are selected from the all UiTM students in that campus.

where the interviewer interview the wrong respondents.

SAMPLE

FRAME

or bias error, where only the interested respondents respond.

SELECTIO

N

As for sampling error happen

because:

As for non-sampling error

happen because:

COVERAGE ERRORMay happen due to unit

in the sample is incorrectly excluded or

included or is duplicate in the sample.

NON-RESPONSE ERRORSome of the respondents might not respond or not

answers the questions being asked.

RESPONSE ERROR Respondents not

understand about the survey or question that

being asked.

REFERENCES http://www.statcan.gc.ca/edu/power-pouvoir/ch6/sampling-echantillonage/521

4807-eng.htm

http://www.qualtrics.com/blog/frequent-sampling-errors/

http://www.abs.gov.au/websitedbs/a3121120.nsf/home/statistical+language+-+types+of+error

http://www.nss.gov.au/nss/home.nsf/NSS/4354A8928428F834CA2571AB002479CE?opendocument

http://srmo.sagepub.com/view/using-published-data/n1.xml

http://stats.oecd.org/glossary/detail.asp?ID=3787

http://support.sas.com/rnd/app/da/new/dasurvey.html

http://stats.oecd.org/glossary/detail.asp?ID=301