Measurement

Why measure? Scientists use a standard method to collect data as well as use mathematics to analyze measurements. We must measure things before we can objectively describe what we are observing, before we can experiment with biological processes, and before we can predict how organisms respond, adjust to, and modify their world.

The Metric System Scientists throughout the world use the metric system to make measurements. The metric system is also used in everyday life virtually everywhere except the United States.

The SI (or Système International d'Unités ) is the modern form of the metric system. Using the same standards of measurement makes it easier for scientists to communicate with one another.

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Metric units commonly used in biology include: meter (m)—the basic unit of length liter (L)—the basic unit of volume kilogram (kg)—the basic unit of mass degree celsius (C)—the basic unit of temperature

The meter (m) is the basic unit of length. Units of area are squared units (two-dimensional) of length. Measured using rulers, meter sticks, etc. Volume is the space occupied by an object. Units of volume are cubed (three- dimensional) units of length. Measured using pipets, graduated cylinders, beakers, etc.

Common Volumes

The kilogram (kg) is the basic unit of mass. Measured using a triple beam balance, electric scale, etc.

Density is mass per unit volume. D=m/v Measured using graduated cylinders, triple beam balances, etc.

Temperature is the measure of the kinetic energy of molecules—that is, the amount of heat in a system. Biologists measure temperature using a thermometer or a temperature probe. The Celsius scale is based on water freezing at 0⁰C and boiling at 100⁰C.

Temperature Conversion Formulas °F to °CDeduct 32, then multiply by 5, then divide by 9 °C to °FMultiply by 9, then divide by 5, then add 32

Mean= Average – Add up all the numbers in the set and divide by the number of numbers – Ex: 2, 3, 5, 8, 10, 11 Mean= ( )/6 = 6.5

Range is the biggest number in the set minus the smallest number in the set Ex: 2, 3, 5, 8, 10, 11 Range= 11-2 = 9

Experimental Errors and Uncertainty No physical quantity can be measured with perfect certainty; there are always errors in any measurement. This means that if we measure some quantity and, then, repeat the measurement, we will almost certainly measure a different value the second time.

We can take greater care in our measurements to reduce the errors and, thereby, gain greater confidence that our measurements approximate ever more closely the true value.

It is important that we recognize and discuss contradictory or unusual data.

Sources of Experimental Error Systematic Errors are due to identified causes and can be eliminated. Errors of this type result in measured values that are consistently too high or consistently too low. Ex: Instrumental (your thermometer is not calibrated)

Random Errors are fluctuations that cause about half of the measurements to be too high and half to be too low. Sources of random errors cannot always be identified. Ex: you measure wrong

Scientific tools including microscopes, computers, graphing calculators, and probeware allow for the gathering and analysis of data.

Graphing Graphs are pictures that help us understand amounts. These amounts are called data.

Types of Graphs A circle graph is shaped like a circle. It is divided into fractions that look like pieces of pie, so sometimes a circle graph is called a pie graph. Many times the fractional parts are different colors and a key explains the colors.

A bar graph uses bars to show data. The bars can be vertical (up and down), or horizontal (across). The data can be in words or numbers.

A histogram is a special kind of bar graph. The data must be shown as numbers in order.

A line graph shows points plotted on a graph. The points are then connected to form a line.

Independent vs Dependent Variables Y axisY axis X axis

Graphing Examples

Which data point on the graph is probably invalid?

According to the graph, how many mice will be born in week 5 if the trend continues?

Which frog species would be most likely to interbreed?

According to the data in the graph, during which time period did the overall bluegill population decline? a. 1993–1996 b. 1996–1999 c. 1990–1993 d. 1999–2002