how to writeup practical reports(08) (1)

7/29/2019 How to Writeup Practical Reports(08) (1)

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This document is available on BHHS Intranet by clicking on subjects (on a strip near the top of the screen) then Science then Mr Ogle then Year? then ??

PRACTICAL REPORTS

PART 1 HEADINGS

PART 2 VARIABLES

PART 3 - EXPERIMENTAL VALIDITY, RELIABILITY AND

ACCURACY

PART 4 - GRAPHS

PART 5 - ERRORS

PART 6 Unit Prefixes, Scientific Notation, Orders of Magnitude,

and Significant Figures

PART 7 STRAIGHT - LINE GRAPHS

PART 8 WORKING INDIVIDUALLY/WORKING IN TEAMS

Part 9 Risk Assessments

PART 1

Some Headings

Aim: Usually 2 or 3 lines about what you are trying to achieve in this experiment.

Method: Should be in point form

Should be a clear set of instructions starting with verbs.

Diagrams: Not always necessary but often they are not only informative but essential

to explain either the method or the results.

Results: Should be tabulated when possible. Units should be stated in the table where

applicable.

The formula and working should be shown for any calculations that are necessary.

Conclusion: *Have you achieved the aim of the experiment?

*Comment on your results, calculations and graphs.

*Compare theoretical results and experimental results where applicable.*Comment on any sources of error.

*Answer all questions from the Instruction Sheet for the prac using the

correct question numbers.

Other headings that an experimental report may include are "Hypothesis" (just after the aim)

then Equipment List and "Discussion" (just before the conclusion).


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PART 3 - EXPERIMENTAL VALIDITY, RELIABILITY AND

ACCURACY

Validity and Reliability

You now need to evaluate the validity and reliability of your results. Your experimental

results are valid if they measure what you think they are measuring. Suppose you were

measuring a spring constant but you added such a large mass that the spring stretched beyond

its elastic limit. You no longer get a straight line when you graph force against extension.

Your spring constant, if you can get one, would not be correct as your experimental results

are not valid.

Validity can be checked in a number of ways. Discussing the design of the experiment and

the pattern of results with others may help. If your results are not reliable it may also

indicate they are not valid. Results are reliable if you get consistent results with repeated

measurements. Another indication that they are reliable is if the amount of experimental

error is small. If your spring has stretched beyond its elastic limit, then the same weight will

produce different extensions. You will not get consistent results - they are not reliable.

Validity

If an experiment measures what you think it measures, as shown by using a different method

to measure the same variable, then the experiment is valid.

Validity refers to the ability of a particular measurement to give you useful information, to

make a useful prediction about the subject of interest.

Reliability

If an experiment can be repeated under the same circumstances and produce the same result,

then the experiment is reliable.

Reliability of an experiment is improved through repetition.

Some good questions to ask:first-hand information and data Secondary information and data

reliability Have I tested with repetition? How consistent is the information withinformation from other reputablesources? Is the data presented basedon repeatable processes?

validity Does my experimental procedure/designactually test the hypothesis that I want itto? Have all variables been identified andcontrolled?

How was the information gathered?Do the findings relate to thehypothesis or problem?

Accuracy

...the closeness of the agreement between a measurement and a true value of the measure

(e.g. temperature).

An experiment is only an accurate as the measuring device used to take the measurement.

Using a device with a smaller limit of reading will improve the accuracy of the measurements

taken with the device.

Neither reliability nor repeatability are the same as accuracy, for a measurement may be both

reliable and repeatable while being wrong.


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Part 3(cont): Reliability, validity and accuracy what do they mean?

Reliability and validity are two terms that continue to cause problems for students. Students in Stages4 and 5 are required to evaluate evidence for reliability and validity. In Stage 6 they are required to

discuss and explain how they would improve the validity and reliability of a first-hand investigation.They could also be asked about the validity and/or reliability of secondary sources of information.

With reference to first-hand data, the glossary of terms on pp. 76-78 of the Science Years 7-10syllabus defines the terms validity and reliability as follows:

validity offirst-hand data

The extent to which the processes and resultantdata measure what was intended.

reliability offirst-hand data

The degree with which repeated observationand/or measurements taken under identicalcircumstances will yield the same results.

When discussing the accuracy of measurements, a dictionary definition such as that provided in theAustralian Oxford Dictionary is appropriate :

accuracy The exactness or precision of a measurement;relating to the degree of refinement inmeasurement or specification.

First-hand investigations:

In the context of students planning first-hand investigations, issues related to accuracy, reliability andvalidity will impact on the choice of the measuring device and how confident you are about theconclusions drawn from the results of the investigation.

The need for accuracy of data should influence the choice of equipment for conducting first-handinvestigations. Where data is collected, quantified or evaluated, reliability refers to the ability of thedata gathering process to provide results that are consistent and within expected ranges. It isimportant that we encourage students to predict expected results and even predict ranges of data.

Validity relates to whether the measurements you are taking are caused by the phenomena you areinterested in.

The relationship between reliability and validity can be confusing. Measurements and otherobservations can be reliable without being valid. A faulty measuring device can consistently provide awrong value therefore providing reliably incorrect results. However, measurements and observationscannot be valid unless they are reliable and accurate.

It is easier to be confident of your conclusion in any investigation when there are limited variablesinvolved and where these variables are controlled.

It is possible to highlight the difficulties involved in such decisions when you think about the difficulty inestablishing the link between smoking and lung cancer, and the link between mesothelioma andasbestos dust. The more complex the situation in terms of the range of potential variables that need tobe controlled and the difficulty in controlling these variables, the less certain it can be that one test willdeliver a valid and reliable answer.


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Harvey Version: ACCURACY, RELIABILITY AND VALIDITY

The Board of Studies definitions are very brief and the following expanded definitions may

be of use:

a) ACCURACY:Exactness or conformity to truth.

Science texts refer to accuracy in two ways:

(i) Accuracy of a result or experimental procedure can refer to the percentage difference

between the experimental result and the accepted value. The stated uncertainty in an

experimental result should always be greater than this percentage accuracy.

(ii) Accuracy is also associated with the inherent uncertainty in a measurement. We canexpress the accuracy of a measurement explicitly by stating the estimated uncertainty or

implicitly by the number of significant figures given. For example, we can measure a small

distance with poor accuracy using a metre rule, or with much greater accuracy using a

micrometer. Accurate measurements do not ensure an experiment is valid or reliable. For

example consider an experiment for finding g in which the time for a piece of paper to fall

once to the floor is measured very accurately. Clearly this experiment would not be valid or

reliable (unless it was carried out in vacuum).

b) RELIABILITY: Trustworthy, dependable.

In terms of first hand investigations the Board seems to define reliability as repeatability or

consistency. If an experiment is repeated many times it will give identical results if it is

reliable. In terms of second hand sources reliability refers to how trustworthy the source is.

For example the NASA web site would be a more reliable source than a private web page.

(This is not to say that all the data on the site is valid.) The reliability of a site can be assessed

by comparing it to several other sites/sources.

c) VALIDITY: Derived correctly from premises already accepted, sound,

supported by actual fact.

A valid experiment is one that fairly tests the hypothesis. In a valid experiment all variables

are kept constant apart from those being investigated, all systematic errors have been

eliminated and random errors are reduced by taking the mean of multiple measurements. An

experiment could produce reliable results but be invalid (for example Millikan consistently

got the wrong value for the charge of the electron because he was working with the wrong

coefficient of viscosity for air). An unreliable experiment must be inaccurate, and invalid as a

valid scientific experiment would produce reliable results in multiple trials.


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NOTE - The notes that follow from this point on are Harveys own work.

ERRORS

The two different types oferror that can occur in a measured value are:

Systematic error this occurs to the same extent in each one of a series of measurements eg

zero error, where for instance the needle of a voltmeter is not correctly adjusted to read zero

when no voltage is present.

Random error this occurs in any measurement as a result of variations in the measurement

technique (eg parallax error, limit of reading, etc).

When we report errors in a measured quantity we give either the absolute error, which is the

actual size of the error expressed in the appropriate units or the relative error, which is the

absolute error expressed as a fraction of the actual measured quantity. Relative errors can

also be expressed as percentage errors. So, for instance, we may have measured the

acceleration due to gravity as 9.8 m/s2 and determined the error to be 0.2 m/s2. So, we say the

absolute error in the result is 0.2 m/s2 and the relative error is 0.2 / 9.8 = 0.02 (or 2%).

Note relative errors have no units. We would then say that our experimentally determined

value for the acceleration due to gravity is in error by 2% and therefore lies somewhere

between 9.8 0.2 = 9.6 m/s2 and 9.8 + 0.2 = 10.0 m/s2. So we write g = 9.8 0.2 m/s2. Note

that determination of errors is beyond the scope of the current course.

Consider three experimental determinations ofg, the acceleration due to gravity.

Experiment A Experiment B Experiment C

8.34 0.05 m/s2 9.8 0.2 m/s2 3.5 2.5 m/s2

8.34

0.6% 9.8

2% 3.5

71%

We can say that Experiment A is more reliable (or precise) than Experiment B because its

relative error is smaller and therefore if the experiment was repeated we would be likely to

get a value forg which is very close to the one already obtained. That is, Experiment A has

results that are very repeatable (reproducible). Experiment B, however, is much more

accurate than Experiment A, since its value ofg is much closer to the accepted value.

Clearly, Experiment C is neither accurate nor reliable.

In terms ofvalidity, we could say that Experiment B is quite valid since its result is veryaccurate and reasonably reliable repeating the experiment would obtain reasonably similar


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results. Experiment A is not valid, since its result is inaccurate and Experiment C is invalid

since it is both inaccurate and unreliable.

How do you improve the reliability of an experiment? Clearly, you need to make the

experimental results highly reproducible. You need to reduce the relative error (or spread)

in the results as much as possible. To do this you must reduce the random errors by: (i)using appropriate measuring instruments in the correct manner (eg use a micrometer screw

gauge rather than a metre ruler to measure the diameter of a small ball bearing); and (ii)

taking the mean of multiple measurements.

To improve the accuracy and validity of an experiment you need to keep all variables

constant other than those being investigated, you must eliminate all systematic errors by

careful planning and performance of the experiment and you must reduce random errors as

much as possible by taking the mean of multiple measurements.


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Part 8 WORKING INDIVIDUALLY/WORKING IN TEAMS

Working Individually Working in Teams

Sometimes great ideas come from

individuals when working alone.

Sometimes an individual can

achieve more in less time because

they dont have to discuss and

explain everything that they are

doing.

Sometimes companies do not

want a group of workers to know

company secrets.

Can share ideas.

Can discuss and develop ideas

and arrive at conclusions faster.

Can achieve more by sharing the

workload.

Can specialise in areas that

individuals have expertise in

already.

Can specialise in areas that

individual members have

researched and become expert in.

Individuals would be more likely to

make errors that go uncorrected for

a long time and hence waste time.

When working as a team it is

important that the team members:

1. identify collective goals

2. define and allocate roles fairly

3. communicate within the team


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PART 9: RISK ASSESSMENTS

Risk Assessment

Risk assessment is a process used to help manage the risks to health and safety that may arise

in the workplace. The purpose of risk assessment is to enable decisions to be made about

appropriate control measures that are required to protect the health of the workers who maybe exposed to hazardous substances at work. The assessment procedure enables a distinction

to be made between the hazard of a substance and the risk to health that arises from actual

exposure to the substance through its use at work. The hazard is the potential for a substance

to adversely affect the health of the people in the workplace. For example, the hazard of

cyanides is that they are extremely toxic and a small quantity, if ingested, can cause

death. The risk is the likelihood that a substance will cause illness due to the way it is used in

the workplace. The risk to health usually increases with the severity of the hazard, the

amount used, and the duration and frequency of exposure. Eg, if a cyanide compound is

sealed in a labelled container and stored to minimise the possibility of breakage, the risk is

well controlled even though the chemical is a serious hazard. Exposure occurs if a personcomes into contact with a substance, by breathing it in, getting it onto the skin or into the

eyes, or by swallowing it. Injection through the skin can also occur through syringes or high

pressure spray or grease guns.

The principles of risk assessment are: identify the hazards, assess risks and control risks.

Identifying the hazards

In order to control the risk from a hazard, it first needs to be identified. Strategies to identify

a workplace hazard include:

observation

carrying out a workplace inspection

knowing the potential hazards associated with the chemical being used in the experiment

investigating complaints analysing accident records.

Knowing the potential hazards associated with chemicals can be achieved by using Material

Safety Data Sheets (MSDS). These sheets usually provide information about the hazard,

routes of exposure, recommended control measures and other actions to prevent or minimise

risks. Your teacher may show you the MSDSs associated with the chemicals you will use in

your experiment.

Assessing the risks

Once hazards have been identified, the next step is to assess their significance. To assess

whether a hazard poses a significant risk you need to ask questions such as:

what is the likelihood of the hazard causing an injury?

what would be the severity of such an injury?

what might be the frequency of injuries occurring?

Controlling the risks

After assessing that a significant risk exists, you need to think about control measures, ie how

to reduce the likelihood of an accident occurring as a result of the hazard. You need to ask

questions such as:

can the hazardous situation be completely eliminated?

can a less hazardous activity be substituted?

if not, can procedures or measures be instigated to reduce the effect of the hazard?

are there controls already in place?

how effective are they?

are more effective or additional controls needed?

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