OUTLINE: sampling and census sampling surveys, frame, size probability and non-probability...

OUTLINE: sampling and census sampling surveys, frame, size probability and non-probability sampling

methods census

OUTLINE: sampling and census sampling surveys, frame, size probability and non-probability sampling

methods census

SSAMPLINGAMPLINGSSAMPLINGAMPLING

collection methods for data

SamplingSampling

any data collection that is not a controlled experiment

i.e. percentage of greenhouse gases in atmosphere above Winnipeg

SSAMPLING AND AMPLING AND CCENSUSENSUSSSAMPLING AND AMPLING AND CCENSUSENSUS

CensusCensus

survey whose domain is the characteristics of an entire population

any study of entire population of a particular set of ‘objects’.

i.e. female polar bears in western Hudson Bay

human residents of Heidelberg

the number of Epacris impressa plants on a single hillside in Riding Mountain National Park


collect, analyse or study only some members of a population then we are carrying out a surveysurvey

aim is to make observations at a limited number of carefully chosen locations that are representative of a distribution

use sample to predict the overall character of the population – accuracy will depend on quality of sample


done for several reasons:

costs less than a census of the equivalent population

they are carried out to answer specific questions,

sample survey will usually offer greater scope than a census (larger geographical area, greater variety of questions)

SSAMPLING AMPLING SSURVEYSURVEYSSSAMPLING AMPLING SSURVEYSURVEYS

development of sampling survey:

state objectives of survey

define target population

define data to be collected

define the required precision and accuracy

define the measurement ‘instrument’

define the sample frame, sample size and sampling method, then select the sample


process of generating a sample requires several critical decisions to be made:

sample frame

sample size

sampling method

errors will compromise the entire survey


if frame is wrongly defined, sample may not be representative of the target population.

frame might be ‘wrong’ in three ways:

contains too many individuals (membership is under-defined)

contains too few individuals (membership is over-defined)

contains the wrong set of individuals (membership is ill-defined)

SAMPLE FRAMESAMPLE FRAME

Two-stage process:

divide the target population into sampling units

i.e. households, trees, light bulbs, soil samples, cities, individuals

create a finite list of sampling units that make up the target population.

i.e. names, addresses, identity numbers, # of 50 mL sample bottles

SSAMPLE AMPLE FFRAMERAMESSAMPLE AMPLE FFRAMERAME

member of a sample/sample frame

in geomatics – points, lines (transects) and areas (quadrats)

i.e. measuring snow depth at 10 cm intervals along a 10 m line

measuring all features that fall within 10 m of a line

SSAMPLING AMPLING UUNITSNITSSSAMPLING AMPLING UUNITSNITS

quantity is not better than quality

in statistics – sample size of 30 or greater is ideal

in geomatics – appropriate sample size is directly related to a distribution’s variability

SSAMPLE AMPLE SSIZEIZESSAMPLE AMPLE SSIZEIZE

aim is to obtain a sample that is representative of the target population.

when selecting a sampling method, we need some minimal prior knowledge of the target population

how we actually decided which sampling units will be chosen makes up the sampling method.

SSAMPLING AMPLING MMETHODETHODSSAMPLING AMPLING MMETHODETHOD

most sampling methods attempt to select units such that each has a definable probability of being chosen - probability sampling methods.

we can ignore probability of selection and choose samples on some other criterion – non-probability sampling methods.

SSAMPLING AMPLING MMETHODETHODSSAMPLING AMPLING MMETHODETHOD

NNON-PROBABILITY ON-PROBABILITY SSAMPLINGAMPLINGNNON-PROBABILITY ON-PROBABILITY SSAMPLINGAMPLING

units that make up the sample are collected with no specific probability structure in mind

i.e. units are self-selected

units are most easily accessible

units are selected on economic grounds

units are considered to be typical of pop’n

units are chosen without an obvious design

considered inferior to other method - no statistical basis upon which the success of sampling method can be evaluated.

may be unavoidable – regard as a ‘last resort’ when designing a sample scheme.

NNON-PROBABILITY ON-PROBABILITY SSAMPLINGAMPLINGNNON-PROBABILITY ON-PROBABILITY SSAMPLINGAMPLING

basis is the selection of sampling units to make up the sample based on defining the chance that each unit in the sample frame will be included

i.e. have 100 students, need 10 to fill out a survey, each student has a 1 in 10 chance or being selected (probability of selection is 0.1)

PPROBABILITY ROBABILITY SSAMPLINGAMPLINGPPROBABILITY ROBABILITY SSAMPLINGAMPLING

each time we apply the same method to the same frame, we will generate a different sample

concerned with probability of each sample being chosen, rather than with the probability of choosing individual units

number of probability sampling strategies


Simple random sampling simplest way

select n units such that every one of the possible samples has an equal chance of being chosen

generate a sample by selecting from the sample frame by any method that guarantees that each sampling unit has a specified probability of being included

how we do the sampling is of no significance (I.e. random number tables, dice, …)


Simple random sampling


i.e. 94407382

94409687

93535459

94552345

94768091

93732085

94556321

94562119

93763450

94127845

94675420

94562119

93763450

94127845

Use random number table to generate six random number between 1 and 14

4, 6, 7, 9, 11, 13


Stratified Sampling used when you suspect the target population

actually consists of a series of separate ‘sub-populations’

stratification is the process of splitting the sample to take account of possible sub-populations

stratified sampling – total pop is first divided into a set of mutually exclusive sub-pops/strata

sub-populations may be of equal sizes or not depending on their relative sizes


Stratified Sampling within each strata, select a

sample usually ensuring that the probability of selection is the same for each unit in each sub-pop – stratified random sample

i.e. national polls and rating surveys


i.e. 94407382

94409687

94535459

94552345

94768091

94732085

94556321

93562119

93763450

93127845

93675420

93562119

93763450

93127845

First split pop into sub-pops (based on the second number in this example)

Then sample from these sub-pops (three from each using a random number table – 1, 2, 5)


Systematic Sampling decide sample size from

the population size; population has to be organized in some way

i.e. points along a river, simple numerical order

simpler in design and easier to administer


Systematic Sampling choose a starting point along the sequence

by selecting the rth unit from one end of the sequence

then take the rest of the sample by a number to r


i.e. 94407382

94409687

94535459

94552345

94768091

94732085

94556321

93562119

93763450

93127845

93675420

93562119

93763450

93127845

First order the sample units (in this case decreasing numerical order)

Next, select the first point (r value) – 2

Then take every third sample after this (2, 5, 8, 11, 14)


CCENSUSENSUSCCENSUSENSUS

aim is to identify and record all members of a population

most countries routinely carry out a census on its population

i.e. Canada – performs a census every 5 years (1981, 1986, 1991, 1996, 2001)

original function to enumerate for electoral purposes, but encompasses a large range of information about national populations

collects important information about the social and economic situation of people living in an area Population Counts

Age, Sex, Marital Status, Families (number, type and structure)

Structural Type of Dwelling and Household Size

Immigration and Citizenship, Education, Mobility, Migration

Mother Tongue, Home Language and official/Non-Official Languages

Ethnic Origin and Population Group (visible minorities)

Labor Market Activities, Household Activity, Place of Work and Mode of Transportation

Sources of Income, Total Income and Family and Household Income

Families: Social and Economic Characteristics, Occupied Dwellings and Household Costs


disadvantages of census:

time consuming - require years of planning

laborious - requires thousands of workers/volunteers

costly - millions of dollars to survey everyone


Errors in census data:

people respond dishonestly due to lack of confidence in confidentiality

full accounting of residences is difficult to document (i.e homeless)

recruiting substandard people to conduct surveys


a census consists of “enumeration” data counts tabulated or ‘aggregated’ by

geographic areas

census regions/enumeration areas are not distributed uniformly and vary in shape, size and orientation

Canada divided into 51,500 enumeration areas

census regions are defined by political boundaries and natural and cultural landmarks

CCENSUS ENSUS RREGIONSEGIONSCCENSUS ENSUS RREGIONSEGIONS

Enumeration Area (EA) smallest reported census area

canvassed by one census representative

125-440 dwellings, depending on situation in rural/urban area

Census Tract (CT) represent urban or rural communities in CMAs and Cas

populations range between 2,500 - 8,000

Census Subdivision (CSD) term applied to municipalities or equivalent


Census Division (CD) areas intermediate between municipality (CSD) and province

level

represent counties, regional districts, regional municipalities

Census Metropolitan Area/Census Agglomeration (CMA/CA) CMA and CA are very large urban cores together with adjacent

integrated urban and rural areas

urban core population >100,000 for CMA, >10,000 for CA

CMA may be combined with adjacent CAs to form ‘consolidated CMA’

Federal Electoral Districts (FED) area entitled to elect a representative member to the House


aggregate census information within the boundaries of the data collection regions.

reduce costs

confidentiality

GIS concerns

census region totals are more abstract and spatially inaccurate

mask the true nature of population distribution


aggregated data reported as census region totals – data presentation is a count by region

also report census totals at region centroids

center of area – balance point for census region shape

center of population – averaging x and y coordinates of the individual pop`n.

RREPORTING EPORTING MMETHODETHODRREPORTING EPORTING MMETHODETHOD

Map of Census divisions

census represents a very important source of data for GIS because:

it provides data of use in many areas of human geography: social, economic, political

the census goes back to Confederation, so historical analyses can be performed

the census provides data in a large variety of readily-mapped spatial zones (eg CMA, county)

CCENSUS AND ENSUS AND GISGISCCENSUS AND ENSUS AND GISGIS

OUTLINE: sampling and census sampling surveys, frame, size probability and non-probability...

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