Collecting data

In a nutshell

Scientific data must be collected in a logical, accurate and precise manner.

Sample size 

A sample size is how big the experimental sample is. Bigger sample sizes are more representative and allow anomalies to be detected. An anomaly is a result which does not fit into the wider pattern of other results in the data. Representative means a small sample which represents a larger sample.

Example 

A scientist investigates the incidence of diabetes among $$50$$​ students. The sample size is therefore $$50$$​.

Example

A scientist investigates the incidence of heart disease in a population of $$100,000$$​ people. The scientist uses a sample size of $$5000$$​ people. In this investigation, $$5000$$​ people in the sample is representative of $$100,000$$ people in the wider population.

Another scientist carries out the same investigation with a bigger sample size of $$100,000$$​ people. This sample size is more representative of the population and allows more opportunities for possible anomalies to be detected.

Accuracy and precision 

Definition

Accuracy is how close a measured value is to its true value. Precision is how close two or more measured values are to each other.

Example 

A mug of water is warmed up to $$50\degree C$$. Student A measures the temperature of water using a thermometer. Student A dips the thermometer down to the bottom of the mug, which gives a reading of $$53 \degree C$$. 

Student A repeats the measurement again, this time immersing the thermometer in the water, without touching the bottom of the mug. This gives a reading of $$51 \degree C$$, which is more accurate because it is closer to the true value of $$50 \degree C$$.

Student A repeats the experiment three times to get the values: $$53\degree C, 51\degree C~and ~50\degree C$$.

Student B repeats the experiment three times to get the values: $$55\degree C, 54\degree C ~and~ 55\degree C$$​.

Student A's data is more accurate than student B's because the measured values are closer to the true value of $$50 \degree C$$​.

Student B's data is more precise than student A's because the measured values are closer to each other.

Equipment

Appropriate equipment must be used in a scientific investigation. Advantages and disadvantages of different equipment are evaluated to choose the most appropriate option. 

Common laboratory equipment in chemistry are listed below.

Example

• pH indicators and pH meters are used to measure pH
• pH indicators change colour, depending on whether a solution is acidic, alkaline or neutral
• A pH meter gives a numerical value of the pH
• If the requirement of the experiment is to determine whether a solution is acidic, alkaline or neutral, a pH indicator may be the most appropriate option because pH indicators are cheaper and easier to use.
• If the investigation requires an exact value of the pH, a pH meter would be the more appropriate option
• As long as the pH meter has been looked after properly, the measurement should be accurate

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Errors

An error is the difference between the measured value and true value.

Anomalies

An anomaly is a result that does not fit in with the pattern in the rest of the data. When processing data and making conclusions, anomalies are not included.

Example 

Student A measures the height of student B using a metre ruler and records her results in the table below to three significant figures: 

The fourth repeat is an anomaly as it does not fit in with the rest of the data. This anomaly was likely due to a random error, such as starting the measurement from the wrong place. Therefore, student A did not include this result when calculating the mean height of student B:

​$${(170+171+171+172)\over 4 }=\underline{ 171~ cm}$$​​