Fieldwork and skills, Year 10: graphs, data maps, statistics and argument

A divided bar chart (also called a compound or stacked bar chart)

提示Each column is cut into coloured slices that add up to the whole.

为什么An ordinary bar chart gives one value per bar. This type splits every bar into its parts, usually as percentages adding to 100, so both the total and the make-up can be compared across places. Its weakness is that the upper slices do not start from the same level, so they are harder to compare by eye.

birth; life expectancy

提示The bottom bars are the youngest children; the top bars are people in their eighties and nineties.

为什么A population pyramid is two bar charts back to back: males on one side, females on the other, with age groups up the middle from youngest to oldest. Its shape is read from the bottom up. A broad base means many babies. Sides that narrow quickly mean many people die young. Sides that stay nearly vertical mean most people survive into old age.

A dispersion graph

提示It is named after how scattered or bunched the values are.

为什么Each site or sample gets its own column of dots, plotted against the vertical scale. You can then see the range, pick out the median and quartiles by counting dots, and spot anomalies. It suits comparing sets of measurements, such as pebble sizes at three river sites.

histogram

提示Its bars touch, because one class runs straight into the next.

为什么A bar chart is for separate categories, such as types of shop, so its bars have gaps. Measurements such as length, depth or speed run on a continuous scale, so the bars for neighbouring classes touch. AQA Geography only asks for classes of equal width, so the height of each bar is simply its frequency.

A pie chart shows shares of one whole, not change over time

提示Ask what the full circle stands for.

为什么The full circle of a pie chart is 100% of something at one moment, cut into slices. It has no axis for years. A line graph suits a value that changes through time, because the slope of the line shows how fast it is rising or falling.

An isoline

提示The first three letters come from the Greek for "equal".

为什么Contours are the best-known example: they join points of equal height. Isobars on a weather map join equal air pressure. Any place between two of these lines has a value between the two figures, and lines drawn close together show the value changing quickly over a short distance.

The variation within it

提示One county may hold both a crowded city and empty moorland.

为什么A choropleth map gives each area one shade for one figure, usually an average. It is quick to read and good for comparing areas. But it suggests that every part of an area is the same, and that the value changes sharply at each boundary, when neither is true.

Desire lines

提示Each one shows only where a journey starts and ends, not the route.

为什么This kind of line is drawn straight from origin to destination. Together they show how far and from which directions people travel, which gives a shop or town its sphere of influence. The other kind follows the real route and changes thickness with the amount of movement.

width

提示A motorway would be shown fat and a country lane thin.

为什么A flow line map shows both the route and the volume of a movement. The key gives a scale, for example 1 mm of thickness for every 100 vehicles an hour. Arrowheads can show direction. Where many thick lines meet, such as in a city centre, they can overlap and become hard to read.

A proportional symbol map

提示The size of each marker is scaled to the figure it stands for.

为什么The marker can be a circle, a square or a bar, placed at the spot the figure belongs to. It shows a value and its location together. Its weaknesses are that big markers overlap in crowded areas, and that exact values are hard to read off without a clear key.

16

提示Find the value a quarter of the way along the ordered list, and the value three-quarters of the way along.

为什么With 11 values the median is the 6th (25 mm). The lower quartile is the 3rd value, 18 mm, and the upper quartile is the 9th, 34 mm. Inter-quartile range = 34 − 18 = 16 mm. Both usual school methods agree for 11 values: the (n + 1) ÷ 4 position, and the median of each half with the middle value left out. It tells a geographer how varied the middle half of the pebbles is, ignoring the extremes.

28

提示Compare the size of the drop with the starting figure.

为什么Change = 250 − 180 = 70. Percentage decrease = change ÷ original × 100 = 70 ÷ 250 × 100 = 28%. Always divide by the earlier figure. A percentage lets a geographer compare changes in places of different sizes fairly: a drop of 70 people matters more on a quiet street than on a very busy one.

2 to under 4 m

提示Look for the group that holds the most sites.

为什么When data is grouped, single values are lost, so there is no single mode. The modal class is the group with the highest frequency: here 2 to under 4 m, with 8 of the 20 sites. It tells you which width band is most typical along the river.

The numbers are tiny, so a change of one gives a huge percentage

提示Look at how many events the claim really rests on.

为什么(2 − 1) ÷ 2 × 100 = 50%, so the sum is right. But with only two events, one fewer may be pure chance. A percentage with no raw figures hides how small the numbers are. Other tricks to watch for are a hand-picked start year, a vertical axis that does not start at zero, and a figure quoted with no comparison.

It was wetter than about 90% of the Julys on record

提示Picture every recorded month lined up from driest to soggiest and split into a hundred equal parts.

为什么Percentiles divide an ordered data set into 100 equal parts. The 90th percentile is the value with about 90% of the data below it, so only about one July in ten was wetter. The median is the 50th percentile and the quartiles are the 25th and 75th.

A judgement

提示Think of what a referee or a court has to deliver at the end.

为什么AQA's list says to describe is to set out characteristics and to explain is to set out reasons. Assess, evaluate and "to what extent" all ask you to weigh things up and decide: how important, how successful, how far. An answer that only describes and explains cannot reach the top level, however accurate it is.

Support it with evidence

提示A detective needs this before naming a suspect.

为什么To justify is to make a case. State the choice, then back it with specific facts and figures from the resources and from your own knowledge, and show how each one leads to the choice. A decision with no backing is only an opinion.

The other side: the scheme's failures or limits

提示A set of scales needs something in each pan.

为什么"To what extent" asks how far something is true, so the answer needs weight on both sides before it decides. A strong answer makes a point, backs it with evidence, explains it, does the same for the opposing view, and then gives a conclusion that follows from what has been weighed.