Misusing “which” and “that” can:
❌ Change the meaning of a sentence, leading to misinterpretation.
❌ Make writing ambiguous, making it difficult for reviewers and readers to understand.
❌ Lower the credibility of your research, potentially affecting publication chances.
This guide will break down the grammatical rules, provide numerous examples, address common mistakes, and offer practical tips to ensure your research paper remains clear and professional.
Understanding the Core Difference: Restrictive vs. Non-Restrictive Clauses
“That” introduces restrictive (essential) clauses.
“Which” introduces non-restrictive (extra) clauses.
A restrictive clause (also called a defining clause) is necessary for identifying the noun it describes. A non-restrictive clause (also called a non-defining clause) adds extra information that can be removed without affecting the sentence’s core meaning.
Key Rule:
✔️ Use “that” for restrictive clauses (no commas).
✔️ Use “which” for non-restrictive clauses (with commas).
When to Use "That" in Scientific Writing
The sentence would not make complete sense without it. These clauses do not use commas.
Examples in Research Papers:
📌 “The genes that regulate cell division are crucial for cancer research.”
✅ Correct! The clause “that regulate cell division” is necessary because it specifies which genes are crucial.
📌 “We analyzed the samples that were collected from patients with diabetes.”
✅ Correct! The clause “that were collected from patients with diabetes” is essential because it identifies a specific subset of samples.
📌 “The software that detects mutations was updated for accuracy.”
✅ Correct! Removing “that detects mutations” would make the sentence unclear because it wouldn’t specify which software was updated.
🚫 Incorrect Use of “That”
❌ “The experiments, that failed, were repeated.”
🚨 Wrong! The clause “that failed” is extra information, not essential.
✅ Correction: “The experiments, which failed, were repeated.”
When to Use "Which" in Scientific Writing
The sentence would still be clear even if you removed it. These clauses must be set off by commas.
Examples in Research Papers:
📌 “The new algorithm, which reduces processing time, improves efficiency.”
✅ Correct! The phrase “which reduces processing time” is extra information. The sentence would still make sense without it:
👉 “The new algorithm improves efficiency.”
📌 “The findings, which were unexpected, led to further research.”
✅ Correct! The phrase “which were unexpected” is an added detail, not crucial to the sentence’s meaning.
📌 “The microscope, which was calibrated last month, produced clearer images.”
✅ Correct! The calibration detail is useful but not necessary to understand the sentence.
🚫 Incorrect Use of “Which”
❌ “The methods which were used in the study are outlined in Section 3.”
🚨 Wrong! The clause “which were used in the study” is essential to identify which methods were outlined.
✅ Correction: “The methods that were used in the study are outlined in Section 3.”
Key Differences Between "Which" and "That"
| Feature | That (Restrictive Clause) | Which (Non-Restrictive Clause) |
|---|---|---|
| Function | Essential for meaning | Extra, non-essential detail |
| Can be removed? | ❌ No | ✅ Yes |
| Punctuation | ❌ No commas | Always use commas |
| Example sentence | "The cells that were exposed to UV light mutated." | "The cells, which were exposed to UV light, mutated." |
| Effect of removing the clause | Changes the meaning | Sentence still makes sense |
Common Errors & How to Avoid Them
🚫 Using "Which" Instead of "That" (Restrictive Clause Error)
❌ “The protein, which binds to the receptor, was studied in detail.”
🚨 Wrong! “Binds to the receptor” is necessary to define which protein was studied.
✅ Correction: “The protein that binds to the receptor was studied in detail.”
🚫 Using "That" Instead of "Which" (Non-Restrictive Clause Error)
❌ “The equipment that was recently calibrated performed well.”
🚨 Wrong! If the calibration is extra information, you should use which with commas.
✅ Correction: “The equipment, which was recently calibrated, performed well.”
🚫 Forgetting Commas with "Which"
❌ “The results which were unexpected led to further analysis.”
🚨 Wrong! The phrase “which were unexpected” is extra information and needs commas.
✅ Correction: “The results, which were unexpected, led to further analysis.”
🚫 Using Commas with "That"
🚨 Wrong! The clause is essential, so do not use commas with that.
✅ Correction: “The techniques that were used in this experiment were effective.”
Tips for Scientific Writers
If the sentence still makes sense, use which.
If removing the clause changes the meaning, use that.
📌 Restrictive clauses (that) are more common in scientific writing because researchers often need to specify exact conditions, methods, or subjects.
📌 Non-restrictive clauses (which) are used for background information but should be used sparingly in scientific papers to maintain clarity.
📌 For formal academic writing, always follow these rules to ensure clarity and avoid rejection from journals due to poor sentence structure.
Tips for Scientific Writers
✔️ Use “that” when the information is essential.
✔️ Use “which” when the information is additional.
✔️ Never use commas with “that.”
✔️ Always use commas with “which.”
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