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examBoard: Cambridge
examType: IGCSE
lessonTitle: Accuracy, Sample Size and Reliability
Geography isn't just about maps and places – it's also about collecting and analysing data to understand our world better. Whether you're measuring rainfall, surveying land use, or counting pedestrians in a town centre, you need mathematical skills to make sure your work is accurate and meaningful.
Key Definitions:
Imagine you're measuring the width of a river for a geography project:
Accurate but not precise: Your measurements are 10m, 12m, 8m, 11m (average 10.25m) when the actual width is 10m.
Precise but not accurate: Your measurements are 15.1m, 15.0m, 15.2m, 15.1m (very consistent but all wrong).
Both accurate and precise: Your measurements are 10.1m, 10.0m, 9.9m, 10.0m (consistent and close to the true value).
In your IGCSE Geography exams and coursework, you'll need to:
Accuracy is crucial in geographical research. When measurements aren't accurate, conclusions can be completely wrong!
Even with the best intentions, several factors can affect the accuracy of your geographical data:
Faulty or poorly calibrated equipment can give incorrect readings. For example, a worn tape measure might stretch, or a rain gauge might leak.
Mistakes in reading instruments, recording data, or calculating results. For instance, misreading a compass bearing by just a few degrees can lead to significant location errors.
Weather conditions, time of day, or seasonal changes can affect measurements. For example, wind can affect sound level readings in an urban noise pollution study.
When measuring river discharge (the volume of water flowing past a point), even small errors in measuring the river's width, depth, or flow velocity can lead to significant inaccuracies in the final calculation. If a river is 10m wide and you measure it as 11m, that's already a 10% error before you've even considered depth or velocity!
Sample size refers to how many measurements, observations, or participants you include in your study. It's a balancing act between practicality and statistical validity.
If your sample is too small, your results might not represent the whole population or area you're studying. This is called sampling error.
Example: Interviewing only 5 people about shopping habits in a city of 200,000 people would give very unreliable results.
Very large samples can be time-consuming and expensive to collect, without necessarily improving your results significantly.
Example: Measuring beach profiles every 1m along a 2km coastline (2,000 measurements) would be excessive when every 50m might give sufficient detail.
For IGCSE Geography, you don't need complex statistical formulas, but you should consider:
When studying land use in a small town, a student decided to survey every 10th building along the high street. This gave them 25 buildings to classify, which was manageable in the time available while still providing a representative sample. They found that 40% of buildings were retail, 30% were services, 20% were residential and 10% were vacant. To check reliability, they asked a classmate to repeat the survey independently and got similar results.
Reliability is about consistency - would you get the same results if you repeated your study? There are several ways to improve reliability in your geographical investigations:
Take multiple readings and calculate an average. For example, measure river depth three times at each point and use the mean value.
Use the same techniques, equipment and procedures throughout your study. For example, always measure pedestrian counts at the same times of day.
Have different people collect the same data independently to check for consistency. This helps identify any observer bias.
There are several ways to test if your data is reliable:
A class investigating microclimates in their school grounds collected temperature readings at 10 locations. To ensure reliability, they took readings at each location three times (at 9am, 12pm and 3pm) and on three different days (Monday, Wednesday, Friday). They also used two different thermometers at each location to check their equipment was working correctly. This approach allowed them to identify consistent patterns rather than one-off anomalies.
Let's look at how these mathematical skills apply to real geographical fieldwork situations you might encounter in your IGCSE studies:
Accuracy: Measure beach profiles using ranging poles and clinometers, ensuring equipment is calibrated.
Sample Size: Collect pebble measurements at regular intervals along a transect line.
Reliability: Repeat measurements at the same locations during different tidal conditions.
Accuracy: Use standardised environmental quality surveys with clear criteria.
Sample Size: Survey enough streets to represent different neighbourhoods.
Reliability: Have multiple students rate the same areas independently to check for consistency.
When conducting geographical investigations, watch out for these common mistakes:
In your IGCSE Geography exams, you might be asked to evaluate the quality of data presented to you. Always consider:
Being able to critically assess data quality is just as important as understanding the geographical concepts themselves!
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