“Claim – Evidence – Reasoning” Statements

When a scientist reports their results, they are telling a story – one that they hope their readers will be able to follow along with and understand both the conclusions, as well as the evidence that they were based on.

When you write a lab report, you are doing the same thing on a smaller scale. Your story will begin with you stating the conclusion you have come to (or the claim you are making). Your writing will then flow into a description of how the evidence you collected supports your conclusion, based on theory-based reasoning that you provide. This flow from your claim to your evidence, then to your reasoning, makes your scientific writing very accessible to your reader.

Let’s look at an example of the use of CER:

Dalton also used data from Proust, as well as results from his own experiments, to formulate another interesting law. The law of multiple proportions states that when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers. For example, copper and chlorine can form a green, crystalline solid with a mass ratio of 0.558 g chlorine to 1 g copper, as well as a brown crystalline solid with a mass ratio of 1.116 g chlorine to 1 g copper. These ratios by themselves may not seem particularly interesting or informative; however, if we take a ratio of these ratios, we obtain a useful and possibly surprising result: a small, whole-number ratio.
$$ \frac{ \color{white}{.}^{1.116 \;g\; Cl}\; / \; \color{white}{.}_{1 \;g\; Cu }} { \color{white}{.}^{0.558\;g\;Cl}\;/\; \color{white}{.}_{1 \;g\; Cu}}=\frac{2}{1} $$
This 2-to-1 ratio means that the brown compound has twice the amount of chlorine per amount of copper as the green compound.
This can be explained by atomic theory if the copper-to-chlorine ratio in the brown compound is 1 copper atom to 2 chlorine atoms, and the ratio in the green compound is 1 copper atom to 1 chlorine atom. The ratio of chlorine atoms (and thus the ratio of their masses) is therefore 2 to 1.

Quote from: Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD. Early ideas in Atomic Theory. in Chemistry 2e, OpenStax 2019. https://openstax.org/books/chemistry-2e/pages/2-1-early-ideas-in-atomic-theory

The first sentence is an introductory statement that creates flow from the previous paragraph (about a Proust-derived law) to this paragraph (about a Dalton-derived law).

The second sentence is the claim.

This third sentence presents evidence.

The fourth and fifth sentences present the reader with calculations bridging the evidence to reasoning; the reader could perform reproduce these calculations in different ways but still obtain that key 2:1 ratio that is necessary to link back to the claim.

The final two sentences present the author’s reasoning; the author is connecting the evidence and calculations provided back to the claim.

Please note: 

  • the second sentence makes it clear what is being claimed:  “when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers”.  That’s a bold claim! 
  • evidence is then presented in the form of observations:  mass ratios of key compounds are given, then used in calculations that demonstrate the “ratio of small, whole numbers” promised in the initial claim.  Note also that other observations, such as the colour and form (crystalline) of the solids under investigation was also provided.  How convinced would you be that this evidence supported the claim if two white crystalline solids – or two clear and colourless liquids – had been compared? 
  • reasoning is then given:  the 2:1 ratio calculated is consistent with CuCl and CuCl₂ compositions.  We tend to avoid words such as “proved”, because this 2:1 ratio could later be found to be due to some other reason; instead, a clear connection has been made between empirical evidence and a claim, and that claim is the “law of multiple proportions”.  Should evidence ever surface that this law of multiple proportions does not apply to an isolated and characterized compound, then the claim needs to be revisited.  This revisiting of evidence in light of new evidence, and the formulation of new claims, is the process of science. 

For additional perspective, please see The Writing Center (at the University of North Carolina at Chapel Hill): https://writingcenter.unc.edu/tips-and-tools/paragraphs/

This implementation of the Claim-Evidence-Reasoning framework is is built using: Kennedy, K.; Folkes, C.  Dropping Anchor.  Science Scope2018, 42(3), 42–47.