How to write your Conclusion and Evaluation
Use the following headings and make sure that all the underlined terms appear in your
conclusion and evaluation. Follow the attached sample as a guide.
Conclusion
 State your conclusion at the beginning in the following form: “The data did/did
not support the hypothesis that ……”
 Compare your conclusion to a textbook source
(give proper citation with page number).
 Outline the scientific process that explains the conclusion (or that explains the
textbook theory if your hypothesis was unsupported by your data).
Evaluation
 Rank the sources of error in order of overall contribution to experimental error.
 Comment on how well each controlled variable was kept constant.
 For any variable that was not fully controlled suggest an improvement to the
experiment.
 Be sure that all your suggested improvements are realistic, specific and detailed.
 Wherever relevant, suggest a way to improve the precision of the equipment and
measurements
 Comment on the amount of replication – was it sufficient? If not then suggest an
improvement.
 Comment on the levels of treatment. Were there enough data points? Were the
data points in the right range? If not suggest improved levels of treatment.

Example of Exemplary work
Lab Title: Investigating the growth of bean seedlings growing in light of different wavelengths
Conclusion - The data did not support the hypothesis that mongo seedling growth would be greatest under white light
followed by (in order) blue, red, yellow and green light. Although the green and yellow light yielded the least amount of
plant growth, as expected, the plants growing under red light had a greater average increase in stem length than those
growing under white light. A t-test showed this difference to be insignificant (p>0.05). Nevertheless the result was
unexpected and it contradicts my biology textbook, which states that white light contains the full spectrum of visible light
and as such contains all the wavelengths useful for photosynthesis (Campbell1). Therefore either my textbook is wrong or
the present experiment had sources of error that prevented us from supporting the hypothesis. Of the two options, the
second is far more probable: our experiment surely had flaws and I shall therefore consider the relative importance of each
possible source of error.
Evaluation - I shall begin this evaluation by ranking the sources of error from those that I consider to be the most
significant to those that are likely to be the least significant: 1) the temperature was not properly controlled; 2) the light
intensity was not properly controlled; 3) the precise wavelengths of light were not determined; 4) the seedling
characteristics were not properly controlled; 5) the measurement of stem-length lacked precision; 6) the experiment was
not replicated sufficiently; and 7) plants were not randomly assigned to the different treatments.
The variable Temperature
Temperature is a variable that must be kept constant because it can affect the rate of photosynthesis. Enzyme activity is
the mechanism by which temperature affects photosynthesis (Campbell, Pp 186-187). However, in this experiment we did
not use thermometers to confirm that the temperature was kept constant. Moreover, there are reasons to believe that the
temperature varied between plants growing under different colored lights. One cause of the temperature varying is that
care was not taken to ensure that each seedling was placed at an equal distance from the light source. As a result, plants
growing closer to the light bulbs would have experienced higher temperatures than those further away. The high incidence
of seedlings dying during the experiment may have been due to excessive heat from the light bulbs. Another reason that
temperature may have varied is that the experiment was set up in a hallway, where students frequently opened a door to
enter and exit the building. Each time the door was opened warm air would have entered the building. Thus plants
growing closer to the door may have experienced warmer temperatures than those further away.
The variable Light Intensity
Light intensity (the concentration of photons) is a variable that must be kept constant because it affects the rate of
photosynthesis. According to Campbell (p 191) plants use pigments to harvest the energy of photons. When a photon
strikes a pigment, the pigment harnesses the photon’s energy to begin the process of photosynthesis. However, in this
experiment we did not measure light intensity. Moreover, the light intensity probably varied between plants growing
under different colored lights because light spreads out as it travels from its source. Thus, plants placed further away from
the light source would have had less energy for photosynthesis than those closer to the light bulbs.
The variable Wavelength of Light
The wavelength of each light source was not determined. Therefore the levels of treatment are not known precisely. In
other words, we can only say that the light sources were blue, red, yellow and green but we can not say where in the range
of blue light (and red, yellow and green light) our experiment was conducted. Moreover, we don’t know if our green light
was pure green light or if it emerged from a mixture of blue and yellow light. In addition, there was no divider placed
between the plants growing under different light sources. As a result plants growing under a bulb of one color may have
been contaminated by the light of another set up. One more problem is that the door in the hallway where the experiment
was performed contained a window. Thus natural sunlight entered the hallway thereby exposing all plants to the full
spectrum of visible wavelengths. Since the red light set up was closer to the door than the white light set up, the plants
growing under the former set up probably received a greater boost of light energy from the sun than the plants under the
latter set up. This could be the reason why the plants under red light grew more on average than the plants under white
light.
1
Campbell, N.A. 1996. Biology, Fourth Edition. Pp 184-193. The Benjamin/Cummings Publishing Company Inc. New York.
The variable Seedling Characteristics
There was variation in the height of the seedlings when we began the experiment. This natural variation in seedling
growth may be responsible for some of the error in this experiment. Moreover, some seedlings were generally healthier
and more robust than others. However, we did not record the condition of plants at the start of the experiment so it is
possible that the plants growing under one light source were naturally destined to grow better than the plants under
another light source.
Measuring stem length
We did not develop a standardized method for measuring the length of stems. Therefore, some students may have
measured differently than others thereby lowering the precision of our data set.
Experimental design
The experiment was fairly well replicated with 18 seedlings grown at each level of treatment. However, due to a fairly
high death rate our sample sizes were reduced to 12, 17, 15, and 17 in the white, blue, red, and yellow light, respectively.
Another problem is that we did not randomly assign the plants to each treatment. As a result we may have biased the
experiment if we tended to select the healthiest or largest seedlings first (and placed them into the red light set up), which
was the first set up in the hallway.
Suggested improvements to the procedure
Considering the aforementioned sources of error it is necessary to repeat the experiment with the following
improvements:
1. The experiment should be conducted in a locked room so that the opening and closing of the door is kept to an
absolute minimum. In this way we will be better able to control the temperature.
2. Each set up should be self-contained so that there is no cross-contamination of light between the setups. This could be
achieved by growing the plants in large boxes with the light source suspended just below the top of the box and with
aluminum foil covering the top to prevent light from escaping. In this scenario we would need to add a large container
of water to each box to keep the plants from desiccating, to regulate the temperature and to ensure that all the set ups
have the same humidity. We would need to use a low wattage bulb to prevent the plants from over-heating and to
minimize the risk of fire and hazard to students.
3. Each plant should be placed at an equal distance from its light source to control the variables temperature and light
intensity.
4. A thermometer should be placed amongst the seedlings under each light source and checked periodically to ensure
that each set up has the same temperature throughout the investigation.
5. A photospectrometer should be used to measure the proportions of light of different wavelengths emitted from each
light source. And we should study a range of wavelengths within each color to produce an accurate action spectrum
that doesn’t generalize each color on the basis of a single wavelength within the spectrum of that color.
6. The plants should be compared at the start to ensure that they are all of similar height and health. Thus a large surplus
of seedlings should be grown to ensure that we have a sufficient number of similar plants to work with. It would take
too long to quantify the health of seedlings precisely (such as by measuring the thickness of the stems using calipers
and measuring leaf color with a photospectrometer) so we will simply make a qualitative judgment based on the
overall appearance.
7. A standardized method must be developed to ensure that each student measures the stems in the same way. One
method would be to remove the seedlings from the soil, rinse them with water, and cut the stem with a razor blade at
the point where the stem begins to loose its green color (i.e., the cut off point between the above ground and below
ground portions of the stem). The stem can be measured from this cut off point to the most apical node.
8. The plants should be randomly assigned to the treatments to remove the possibility of human bias.