Sampling
Obtaining information from only a part of a large population to know about the whole population.
sampling units - representative of the whole population.
OBJECTIVE
Secure a sample which will represent the population and reproduce the important characteristics of the population under study as closely as possible.

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1.Difficult to survey s the entire area
2.cost is too high
3. population is dynamic - population may change over time.
4.More time
The sampling makes the supervision more effective.
The method of sampling is called probability sampling.

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All members of a population have an equal and independent chance of being selected.

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Equal chance of being selected

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Each unit of the population has an equal chance of being selected for the sample.
Simple random sampling is the simplest form of sampling
It is the basis for many other sampling methods.
Simple random sampling is most applicable for the initial survey in an investigation

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Whenever a unit is selected for the sample, all the units of the population are equally likely to be selected.
When the first unit is selected - chance of selection which is 1/N
When the second unit is selected, all the remaining (N-1) units of the population have 1/(N-1)chance of selection.

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Random sampling divided in to strata

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Sampling is to divide a heterogeneous population into sub-populations
Sub population usually known as strata
Each strata is internally homogeneous
Sampling units are selected from stratum.
Method of division of area in to homogenous groups is called stratification

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First selecting large sized units
Then choosing a specified number of sub-units from the selected large units is known as sub-sampling.
The large units are called ‘first stage units’
The sub-units the ‘second stage units’.
The procedure can be easily generalized to three stage or multistage samples.

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The sampling of a forest area may be done in three stages,
1.selecting a sample of compartments as first stage units
2.choosing a sample of topographical sections in each selected compartment
3.taking a number of sample plots of a in each selected topographical section.

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Same sampling units are used at the different phases of sampling
To collect different information or same information by different methods.
Two phase sampling is commonly used in forest inventories
Two-phase sampling is sometimes called “double sampling”.

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The no. of bamboo culms in the forest is to be estimated
In first phase no. of clumps/hec is determined by large systematic survey
In second phase culms/clump is determined by smaller area taken in the first phase.

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Samples are selected according to the subjective judgment of the observer
Basis of certain rules or guidelines indicating what sample should be chosen.

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It is a statistical method involving the selection of elements from an ordered sampling frame A method of selecting according to a random starting point and a fixed, periodic interval

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The most common form of systematic sampling is an equal-probability method
sampling interval k is calculated as:
k - sampling interval
N - population size
n - sample size

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Number of samples are tested one by one, accumulating the results until a decision can be made.

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It is a non-probability sampling technique
Researcher picks a single or a group of sample in a given time interval, conducts his study, analyzes the results
Then picks another sample if needed .
This sampling technique gives the researcher limitless chances of fine tuning his research methods

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Point Sampling The circle represent the cross section of trees at bh.
The lines indicate the critical angle of the instrument projected from the sampling point

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Counting from a random point the number of the trees whose breast height cross section exceeds a certain critical angle
when multiplied by a constant factor gives basal area per hactare

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Point Sampling 1. Basal Area Factor (BAF)
How much BA does each sampled tree represent?
2. Plot Radius Factor
How far away can a tree be and still be sampled?
3. Calculations from samples:
Basal area per acre
Trees per acre
Volume per acre

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Diameter of a tree and its distance to a neighboring tree defined a measurable angle

A series of sampling points are selected randomly or systematically distributed over the entire to be inventoried .
Trees around this point are viewed through any angle-gauge at breast height
All trees forming an angle bigger than the critical angle of instrument are counted.

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Each tree counted regardless of its d.b.h.
represents same basal area
Same basal area but tree is away - Not counted
Same distance from the point but tree is small - Not counted

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Horizontal point sampling is used for determination of the following
1.Basal area per hectare(BA)
BA per hectare = BAF × n
n= no of tally trees
F = 1/n
BA = basal area

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Tally Half Tally Non Tally

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Knowing the diameter of every tally tree it is possible to determine the number of stems per hectare .
No.of trees/ ha = basal area factor of prism
total basal area of the tally trees

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Volume /hec(V)= Basal area × stand form height
Basal area directly calculated from the point sampling instrument
Stand form height is obtained by volume table

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The line sampling is application of the angle count technique.
Instead of a sample point a sample lines are used
Basal area counts with an instrument of BAF of 1 sq m/ha are conducted on one side of the line.

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Basal area of the sample calculated by
d= dbh in cm of a tree enumerated in the horizontal line sampling
Vertical angle counts on one side along this line.

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The volume in cum/ha can be estimated by
F= stand form factor
d=dbh in cm enumerated by vertical line sampling
No work has been done in the direction in India so for

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The proposed approach for the 2010 Forest Resources Assessment (FRA) of the UN Food and Agriculture Organization (FAO)
satellite monitoring of deforestation rates over large areas include the use of sampling.
It is a systematic sample of 10 km wide squares at every 1° intersection of latitude and longitude.

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systematic samples for estimating deforestation at national, sub-national and continental level
Sampling may reduce the cost of monitoring
A common sampling approach is systematic sampling, in which sample units of a constant size are distributed in some regular manner, such as a grid.

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The cost is lower
Data collection is faster
Easily planned
Faster in carrying out and mostly cheaper
it gives better estimates of the mean than unrestricted random
it will give better accuracy

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1.Sampling methods used for estimating change in forest resources
2.Sampling is used in ecological survey
3.Using point sampling we can estimate
1.Basal area per hectare(BA)
2.Number stem per hectare
3.Volume per hectare

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4.Multistage sampling is used in bamboo
5.Determination of insect population using random sampling
6.systematic sampling used for estimation of deforestation rates
7.Forest resources assessments and monitoring
8. Sampling strategies used in forest soils

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