Plan it, then write it in full sentences
- Know what the six marks are given for, and why there are seven ways to earn them.
- Write one precise sentence for each CORMMSS letter, for any organism.
- Avoid the vague words that examiners ignore, and check your plan in under a minute.
THE FRAMEWORK
C · O · R · M · M · S · S
Change, Organism, Repeat, Measure (what), Measure (when), Same, Same. One sentence for each letter gives you seven creditable points for a six-mark question.
How the question is marked
The question usually starts with a short context, then asks you to design an investigation to find out whether, or how, one factor affects another. It ends by asking for experimental details written in full sentences, and it is worth six marks. Across recent International GCSE Biology papers, mark schemes for these questions follow the same pattern, summarised here in our own words.
The scheme lists seven creditable points, one for each CORMMSS letter (C, O, R, M1, M2, S1, S2), and awards one mark per point to a maximum of six. You can miss one and still score full marks, but aim for all seven so a weak sentence does not cost you.
The question tells you to write in full sentences and include experimental details. A bullet list of single words, or a plan with no quantities, apparatus or method, makes points hard to credit.
Vague words are routinely ignored: “amount” without a unit, “size” when you mean mass, “observe” or “see” instead of a measurement. A named quantity with a unit is what earns the mark.
M1 must measure what the question asks about. A question about yield wants the mass or number of the crop harvested; a question about attraction wants a count of animals in a stated time; a question about decay wants a change in mass.
M2 wants a stated time for measuring. Schemes often expect a sensible minimum: about a week or more for decay or growth, a growing season for crop yield, minutes for enzymes and yeast.
Each letter has a list of acceptable answers, and equivalent wording is allowed. There is rarely one “right” controlled variable; any sensible, different, named factor kept the same can earn S1 and S2.
How to go about it in the exam
Allow about six to eight minutes. Most of the marks are lost in the first minute, by not deciding exactly what is changed and what is measured, so plan before you write.
- Read the stem twice. Underline the factor you must change and the outcome you must measure. The question often names both: “how the concentration of X affects Y”. Note the organism and the scale (a whole plant, a leaf disc, a single layer of cells).
- Name the two variables. Write the independent variable (what you change) and the dependent variable (what you measure) in the margin before anything else. If you cannot name the dependent variable as a number, you do not yet have an M1.
- Fill the planning grid. Spend about a minute jotting one word or number for each letter: values for C, the matching for O, how many for R, apparatus and unit for M1, a time for M2, and two different controlled variables for S1 and S2.
- Write one sentence per letter. Turn each row into a full sentence with a quantity and unit. Seven short, precise sentences beat one long paragraph, and they are easy for the examiner to credit.
- Check against CORMMSS. Run your eye down the answer and tick C, O, R, M, M, S, S. Replace any “amount”, “observe” or “same conditions” with a named quantity before you move on.
CORMMSS, letter by letter
Each letter is one mark point. The good sentences below would be credited; the weak ones show the most common reasons a point is missed.
C Change
The independent variable: what you change, with at least three values or a range, plus a control experiment where it makes sense.
Name the independent variable and give its values. Two values can be enough to earn the mark, but three or more (or a stated range) let you see a pattern, so aim for at least three. When the question asks whether something has an effect at all, include a control experiment without it, such as 0% salt or no fertiliser, so you have something to compare with.
GOOD
- I will water the seedlings with sodium chloride solutions of 0%, 0.5%, 1.0%, 1.5% and 2.0%.
- I will keep the yeast suspensions in water baths at 20 °C, 30 °C, 40 °C and 50 °C.
WEAK
- I will use different amounts of salt. “Amounts” is vague and no values are given, so the examiner cannot see what is changed.
- I will change the temperature. This names the variable but not the values, so it reads as an aim, not a method.
O Organism
The living things you use are the same: same species, and the same age, size, mass or sex.
Living things vary, so a fair test starts with matched organisms. Say they are the same species (or variety, or strain) and give at least one more match: same age, same size or mass, or same sex. For people, match age, sex, mass or fitness. For enzymes, the "organism" point becomes the same source and concentration of enzyme.
GOOD
- All the seeds will be mung beans from the same packet, of a similar mass.
- I will use twenty adult woodlice of the same species, collected from the same log.
WEAK
- I will use some plants. No species and nothing matched, so differences between plants could explain the results.
- I will use the same amount of yeast. “Amount” has no unit. Say the same mass (for example 2 g) or the same volume of a suspension of the same concentration.
R Repeat
Repeat each value (or use several organisms per value) and calculate a mean.
One plant or one tube can be unusual. Use several organisms or replicates for each value, or repeat the whole test, then calculate a mean for each value. Saying how many repeats (for example three dishes per concentration) makes the sentence stronger.
GOOD
- I will set up three dishes of ten seeds for each concentration and calculate the mean percentage that germinate.
- I will repeat the test three times at each temperature and work out the mean volume of gas.
WEAK
- I will make sure it is reliable. This states a hope, not an action. Say what you repeat and that you calculate a mean.
M1 Measure 1: what
The dependent variable and exactly how you measure it, with the apparatus or unit.
Say what you measure and how: count, weigh, time, or read a scale, with the apparatus and unit. Measure what the question asks about. If the question is about yield, measure the mass of the crop or seeds harvested, not the height of the plants. "Observe" or "see which is best" earns nothing because there is no measurement.
GOOD
- I will count how many seeds in each dish have a root at least 2 mm long, and calculate the percentage that germinated.
- I will collect the carbon dioxide in a gas syringe and read the volume in cm³.
WEAK
- I will observe which plants grow best. “Observe” gives no measurement, and “grow best” is not defined.
- I will measure the height of the plants to find the yield. Height is not yield. Yield is the mass (or number) of the useful product harvested.
M2 Measure 2: when
When you measure, or over what time: a stated time that suits the process.
Give a stated time for the measurement, the same for every value. It must suit the biology: minutes for an enzyme or yeast, days for germination, weeks for growth, a whole growing season for crop yield. "After a while" or "until it finishes" does not earn the mark.
GOOD
- I will count the germinated seeds after five days.
- I will read the syringe after 10 minutes in the water bath.
WEAK
- I will leave them for a while and then check. No stated time, so measurements could be taken at different times.
- I will weigh the tomatoes after one day. A time is stated, but it is far too short for fruit to form, so it does not suit the process.
S1 Same 1
A first controlled variable, named precisely, and how you keep it the same.
Name a variable that could affect the dependent variable and say how you keep it the same: a stated volume, temperature, light source or equipment. Quantities need units. "Same amount of water" is usually ignored; "10 cm³ of solution in each dish" is credited.
GOOD
- Each dish will receive 10 cm³ of solution, measured with a measuring cylinder.
- I will keep all the dishes in an incubator at 25 °C.
WEAK
- I will keep everything the same. No variable is named, so nothing can be credited.
- I will give them the same amount of water. “Amount” without a volume and unit is not precise enough.
S2 Same 2
A second, different controlled variable, also with how it is kept the same.
A second, different controlled variable earns the second S mark. It must be a different factor from S1 and from the independent variable. Good choices depend on the organism: light and carbon dioxide for plants, food and space for animals, pH and substrate concentration for enzymes.
GOOD
- The dishes will sit the same distance from the same lamp, so they receive the same light intensity.
- Each tube will contain 10 cm³ of 5% glucose solution.
WEAK
- I will keep the temperature the same. Credited only if temperature is not the variable you are changing, and stronger with how (a stated temperature, a water bath).
Common pitfalls, and the fix
These are the errors examiner reports return to year after year. Each fix turns a sentence that earns nothing into one that earns a mark.
“Same amount” with no quantity
Weak: I will give each plant the same amount of water.
Better: I will give each plant 50 cm³ of water every two days, measured with a measuring cylinder.
“Observe” with no measurement
Weak: I will observe which tank has the healthiest fish.
Better: I will weigh each fish at the start and after four weeks and calculate the mean change in mass.
Height used as yield
Weak: I will measure the height of the wheat to compare yield.
Better: At harvest I will cut the ears, dry them and weigh the grain from each plot.
Not naming the independent variable
Weak: I will set up some tubes and compare them.
Better: I will change the pH using buffer solutions of pH 4, 5, 6, 7 and 8.
Asking people for an opinion
Weak: I will ask volunteers which drink tastes sweeter.
Better: I will measure the glucose concentration with test strips or a colorimeter after Benedict’s test.
Vague controlled variables
Weak: I will keep the conditions the same.
Better: I will keep all the trays in the same room at 20 °C and water each with 25 cm³ a day.
Describing the wrong method
Weak: Writing a full potato-cylinder osmosis method when the question was about a single layer of onion cells under a microscope.
Better: Read the stem again and design for the organism and scale it names.
A time that does not fit the process
Weak: I will measure the mass of the logs after one hour.
Better: I will weigh the logs every week for eight weeks.
Three full answers, annotated
These contexts are invented for practice. The letter beside each sentence shows the mark point it earns.
Salt and seed germination
Some farmland near the coast has salty soil. Design an investigation to find out how the concentration of salt affects the germination of mung bean seeds. Include experimental details in your answer and write in full sentences. (6)
- CI will make sodium chloride solutions of 0%, 0.5%, 1.0%, 1.5% and 2.0%, where 0% (distilled water) is the control.
- OAll the seeds will be mung beans from the same packet, chosen to be a similar size.
- RFor each concentration I will set up three Petri dishes, each with ten seeds on filter paper, and I will calculate the mean percentage germination.
- M1A seed counts as germinated when its root is at least 2 mm long, so I will count these seeds and work out the percentage in each dish.
- M2I will do the count after five days.
- S1Each dish will receive 10 cm³ of its solution from a measuring cylinder, with lids on to stop the paper drying out.
- S2All the dishes will be kept in the same incubator at 25 °C in the dark.
Seven sentences, seven letters. The control (0%) sits inside C; the germination rule makes M1 precise.
Woodlice and damp conditions
Woodlice are often found under logs and stones. Design an investigation to find out whether woodlice prefer damp or dry conditions. Include experimental details in your answer and write in full sentences. (6)
- CI will use a choice chamber with one side over water, which makes it damp, and the other side over a drying agent, which makes it dry.
- OI will use ten adult woodlice of the same species, of a similar size, collected from the same place.
- M1M2I will place the woodlice in the centre and, after 10 minutes, count how many are on the damp side and how many are on the dry side.
- RI will repeat this five times with fresh woodlice and calculate the mean number on each side.
- S1S2The chamber will be covered so that both sides are equally dark, and it will be kept in the same room at about 20 °C.
- no markI will handle the woodlice gently with a soft paintbrush and return them to where they were found afterwards.
One sentence can earn two letters when it holds two ideas, as M1 with M2 and S1 with S2 do here. The ethics sentence earns no mark but is good practice, and shows care for the animals.
Temperature and yeast respiration
A baker wants dough to rise quickly. Design an investigation to find out how temperature affects the rate of respiration in yeast. Include experimental details in your answer and write in full sentences. (6)
- CI will place tubes of yeast suspension in water baths at 20 °C, 30 °C, 40 °C, 50 °C and 60 °C.
- OEach tube will contain 10 cm³ of the same yeast suspension, made from one batch of dried yeast of the same variety.
- S1I will add 10 cm³ of 5% glucose solution to every tube so the substrate concentration is the same.
- M1I will connect each tube to a gas syringe and record the volume of carbon dioxide produced in cm³.
- M2I will leave each tube for 5 minutes to reach the temperature, then measure the gas collected over the next 10 minutes.
- RI will repeat each temperature three times and calculate the mean volume.
- S2Every tube will be the same size, and the glucose solution will be made with the same pH buffer.
For microorganisms, O becomes the same strain and the same mass or concentration of cells. Counting bubbles would earn less than a gas syringe because bubbles vary in size.
Plants, animals, microorganisms and enzymes
| Kind | Change (C) | Organism (O) | Measure (M1) | When (M2) | Same (S1, S2) | Watch out |
|---|---|---|---|---|---|---|
| Plants | Light intensity or hours, carbon dioxide, temperature, mineral ions (fertiliser concentration), salt, pH of soil or water. | Same species or variety, same age, seeds from the same batch, seedlings of similar height at the start. | Mass (fresh or dry) of the plant or crop, number of seeds germinated, bubbles or gas volume from pondweed, change in mass for water loss. | Minutes for gas from pondweed; days for germination; weeks for growth; a season for yield. | Light, temperature, carbon dioxide, volume of water, type and volume of compost, fertiliser, pot size. | Measure yield as harvested mass, not height. Dry mass is the fairest measure of growth but kills the plant. |
| Animals (including people) | Temperature, food type, exercise type or duration, a smell or colour presented, humidity or light in a choice chamber. | Same species, age, sex, mass or size; for people, same age, sex, mass or fitness, or the same person on different days. | Count of animals in each zone, change in mass or length, heart rate in beats per minute, breathing rate, reaction time. | Minutes for behaviour or pulse; a week or more for growth or a training programme. | Food type and mass, volume of water or tank size, temperature, light, time of day, number of animals per container. | Ethics: use as few animals as possible, avoid harm or stress, keep them in suitable conditions, return them afterwards, and get consent from people. |
| Microorganisms | Temperature, concentration of an antiseptic or antibiotic, sugar type or concentration, pH. | Same species or strain, same volume or mass of culture, same number of cells. | Clear-zone diameter on agar, volume of carbon dioxide in a gas syringe, change in mass, colony count. | Minutes for gas from yeast; one to three days for agar plates. | Volume and concentration of nutrient or sugar, depth of agar, disc size, temperature (unless it is the variable). | Aseptic technique, seal the lid with tape but do not seal all round, incubate at 25 °C or below, and do not open plates after incubation. |
| Enzymes | Temperature, pH (buffer solutions), substrate concentration, enzyme concentration. | The “O” becomes the same enzyme from the same source, at the same concentration and volume. | Time for starch to disappear (iodine no longer turns blue-black), volume of oxygen from catalase, time for a colour change. | Test at stated intervals, such as every 30 seconds, or for a stated time, such as 5 minutes. | Volume and concentration of substrate, volume of enzyme, pH buffer, temperature (whichever is not changed). | Let solutions reach the water-bath temperature before mixing. Enzymes are denatured, not killed. |
Designing fairly for animals and people
An ethics sentence does not earn a CORMMSS mark, but it can be asked for in its own right, and it shows a well-designed investigation.
- Use the smallest number of animals that still gives repeats and a mean.
- Choose invertebrates or observation instead of vertebrates where you can.
- Do not cause pain, injury or lasting stress, and keep conditions within what the animal normally meets.
- Give food, water and shelter, handle animals gently, and return wild animals to where they were found.
- For people: get informed consent, let them stop at any time, keep exercise and drinks within safe limits, and keep results anonymous.
Quick checklist
- C I named the independent variable and gave at least three values or a range, with a control experiment if the question asks “does it have an effect”.
- O I said the organisms are the same species and matched age, size, mass or sex (or same enzyme source and concentration).
- R I used several organisms or replicates per value, or repeats, and said I would calculate a mean.
- M1 I said exactly what I measure and how (apparatus or unit), and it is what the question asks about.
- M2 I gave a stated time for the measurement that suits the process.
- S1 I named one controlled variable, with a quantity and unit, and how I keep it the same.
- S2 I named a second, different controlled variable and how I keep it the same.
Graph skills: SLAPUK and choosing a scaleGot your results? Plot them with SLAPUK: scale, line, axes, plots, units, key.
Planning grid
| Letter | Ask yourself | Sentence starter |
|---|---|---|
| C | What do I change, and to which values? Is there a control experiment? | I will change … using …, …, … and … . |
| O | Which organism, and how do I match them? | All the … will be the same species and the same … . |
| R | How many per value, and do I calculate a mean? | For each … I will use … and calculate the mean … . |
| M1 | What exactly do I measure, and with what? | I will measure … by … in … (unit). |
| M2 | When, or over what time? | I will take this measurement after … . |
| S1 | Which variable could spoil the test? How do I fix it? | I will keep … the same by … . |
| S2 | A second, different variable to control? | I will also keep … the same by … . |
Practice prompts
Plan each one with the grid and write your answer on paper. Then open the hinted mark scheme, tick off the letters you earned, and rate yourself. Nothing you write is saved; only the red, amber or green rating is kept in this browser.
Ratings are saved in this browser on this device, under a KS4 key kept separate from International A Level recall. Written answers are temporary and cleared when you leave the page. Use Reset to clear the ratings on this device.
A grower thinks that giving tomato plants extra hours of light each day increases the yield of tomatoes. Design an investigation to test this idea. Include experimental details in your answer and write in full sentences. (6)
Hint
Yield means the tomatoes, not the plant. Think about how long tomatoes take to ripen before choosing M2.
Hinted mark scheme (any six of seven)
- C Different hours of light per day, such as 8, 12, 16 and 20 hours, using lamps on timers.
- O Same variety of tomato, plants of the same age or size, or grown from the same batch of seeds.
- R Several plants for each light period, or repeats, with a mean calculated.
- M1 Mass of ripe tomatoes picked from each plant (or number of tomatoes), weighed on a balance.
- M2 A stated time long enough for fruit: for example, picking over eight weeks or until the end of the season.
- S1 Same temperature, for example all in one glasshouse kept at 22 °C.
- S2 Same volume of water, same compost or fertiliser, same carbon dioxide concentration, or the same light intensity.
No credit for:
- Plant height, number of leaves or “how well they grow” for M1.
- “Same amount of water” with no volume.
- A time of a few days for M2.
Some students believe that drinking coffee makes their reactions faster. Design an investigation to find out whether caffeine affects reaction time. Include experimental details in your answer and write in full sentences. (6)
Hint
People are the organism. How can you measure reaction time with a number? Think about what else they have eaten or drunk.
Hinted mark scheme (any six of seven)
- C Drinks with different caffeine content (for example 0 mg, 50 mg and 100 mg in the same volume), with a caffeine-free control.
- O People of the same age, sex or mass, or the same person tested on different days.
- R Several people per group, or several attempts per person, with a mean.
- M1 Reaction time from a ruler-drop test (distance converted to time) or an online timer, in seconds.
- M2 Tested a stated time after the drink, such as 30 minutes.
- S1 Same volume and type of drink (for example 200 cm³ of decaffeinated coffee with caffeine added).
- S2 Same time of day, no other caffeine that day, same hand and same ruler, or the same practice attempts.
No credit for:
- Asking people whether they feel more alert.
- Doses that are unsafe; keep to normal drink amounts and get consent.
A company claims its mouthwash kills bacteria. Design an investigation to find out how the concentration of the mouthwash affects the growth of bacteria. Include experimental details in your answer and write in full sentences. (6)
Hint
Use agar plates and paper discs. What can you measure with a ruler? Remember safe incubation.
Hinted mark scheme (any six of seven)
- C Paper discs soaked in mouthwash diluted to, for example, 0%, 25%, 50%, 75% and 100%, with water as the control.
- O The same species or culture of bacteria, spread evenly with the same volume on each plate.
- R Several discs or plates per concentration, with a mean.
- M1 Diameter of the clear zone around each disc measured with a ruler in mm (or area calculated).
- M2 Measured after a stated time, such as two days.
- S1 Incubated at the same temperature, for example 25 °C (not 37 °C, to avoid growing human pathogens).
- S2 Same size of paper disc, same soaking time, same depth or type of agar.
No credit for:
- “See which plate has fewer bacteria.”
- Counting individual bacteria by eye.
Design an investigation to find out how wind speed affects the rate of water loss from a leafy shoot. Include experimental details in your answer and write in full sentences. (6)
Hint
A fan at different distances, or settings, changes wind speed. Water loss can be measured by mass or with a potometer.
Hinted mark scheme (any six of seven)
- C A fan on different speed settings or at different distances (for example 25, 50, 75 and 100 cm), with no fan as a control.
- O Shoots of the same species with a similar number or area of leaves.
- R Several shoots per wind speed, or repeats, with a mean.
- M1 Distance moved by the air bubble in a potometer, or the decrease in mass of the shoot in a sealed flask, on a balance.
- M2 Over a stated time, such as 10 minutes per reading or 2 hours on the balance.
- S1 Same temperature in the room.
- S2 Same light intensity from the same lamp at the same distance, or same humidity.
No credit for:
- “Watch how quickly the leaves wilt.”
- Changing the shoot between wind speeds without matching leaf area.
The marking summary is ScholarLab’s own description of patterns in past Pearson Edexcel International GCSE Biology mark schemes and examiner reports; no Pearson wording is reproduced. All contexts, model answers and practice prompts are original and illustrative. For real exam-style questions, see KS4 exam practice, or go back to the KS4 Logic Lab or the KS4 course.