GRAMMAR POINT 1 . 0 Structure of the passive

The basic passive is formed using [Verb be +ed].

EXAMPLES
§ Active sentence: I completed the experiment.
§ Passive sentence: The experiment was completed.

You can browse this site to review passive sentence structures if you are not sure about the form.
http://owl.english.purdue.edu/handouts/grammar/g_passive.html

GRAMMAR POINT 1 . 1 Use active sentence structure to put emphasis on methodology decisions

“There is a tendency for passive sentences to indicate routine procedures while active sentences indicate new, deliberately chosen, important, or unexpected procedures” (Swales & Feak, 1994 p. 161).

Use active sentences using “we” or “our” if you want to emphasize YOUR decisions, YOUR method, or YOUR innovation. For standard procedures, generally use the passive structure. In this example, the researchers are explaining why they thought it was important to include a material in their analysis. When they explain their choice they shift to the active tense using we and emphasize that they are doing the analysis by using the key words “our analysis.”

EXAMPLE
The infill material considered was mainly concrete: several concrete qualities were studied in order to investigate the influence of variation of the elastic modulus of the infill; moreover the C20 lightweight concrete (LWC) and polyurethane (PUR) were considered as alternatives. As these two materials, especially PUR, offer the advantage of low weight we thought it worthy to include them in our analysis. In addition, PUR seems to be drawing the attention of other researchers as a possible hollow sections infill material [22].
Source: Thomopoulos and Koltsakis. Journal of Constructional Steel Research 59 (2003) p.671 ©2003
In this example, the authors switch to the active “we fixed” when they are describing their choices in simplifying a procedure.

EXAMPLE
Note that in the present investigation, the loading direction was defined by two angles, i.e., β and ω. Loading angle β ?was varied between 0° and 90° ?and the angle ω was set to 90° ?for all the cases analyzed. For simplicity in setting-up the test, we fixed the axial loading at β=90?° and made starter notches of different inclined angles instead of varying the loading angle β, as shown in the two side elevations of Fig. 1. All the specimens were annealed at 923 K for 1 h before fatigue testing. The fatigue crack was initiated and propagated from the starter notch by subjecting the notched specimen to a constant amplitude, sinusoidal, tension-to-tension loading. This loading was exerted by a 100 kN electro servo-hydraulic MTS testing machine operating at a stress ratio of R=σmin/σmax=0.1, and a frequency of 2.0 Hz.
Source: Bian and Lim: International Journal of Fatigue 25 (2003) 521–531 p.522 ©2003

GRAMMAR POINT 1 . 2 Be careful when using the passive in describing methods or results

These two sentences can have very different meanings in English.
§ The temperature was increased by 50 C.
§ The temperature increased by 50 C.

The passive is used when the researcher changes the conditions of the experiment, but it is not used when a result that is not known before measurement is just observed. In this first example, it is clear that the researcher is constantly manipulating the temperature.

EXAMPLE
The temperature was increased by about 17 °C/min to 850 °C, where it was held constant for 20 minutes and then decreased again by about 17 °C/min. When the temperature had decreased to 650 °C the gas was stopped and vacuum was again achieved. The sample was then allowed to cool naturally back to room temperature.
Source: http://www.phy.ilstu.edu/~intreu/colinProj.htm
However, in this example describing global warming, it is clear that no one can change the temperature of the whole planet! It is just the observed result.

EXAMPLE
Over the last 40 years — which is the period with the most reliable data — the temperature increased by about 0.5°F (0.2-0.3°C).
Source: http://www.teachersdomain.org/9-12/sci/ess/watcyc/lp_global2/index.html
In some papers, failure to distinguish the two grammar points may make the experimental procedure used in a paper unclear, or even funny. Imagine a biomedical engineering paper that was testing a new type of bandage on a group of patients, and the researchers wanted to have a bigger sample to get better results.

EXAMPLE
A. The sample size was increased to a total of 110 patients.
B. The sample size increased to a total of 110 patients.
In example B above, it seems like the patients suddenly cloned themselves and expanded! However, it is a difficult grammar point and you will often find misused passive sentences even in published papers.