Below are extracts from:

Robert A. Day, How to write and publish a scientific paper, Oryx Press, 5th edition, 1998.

Chapter 32

Use and Misuse of English

Tense in Scientific Writing

There is one special convention of writing scientific papers that is very tricky. It has to do with tense, and it is important because its proper usage derives from scientific ethics.

When a scientific paper has been validly published in a primary journal, it thereby becomes knowledge. Therefore, whenever you quote previously published work, ethics requires you to treat that work with respect. You do this by using the present tense. It is correct to say "Streptomycin inhibits the growth of M. tuberculosis (13)." Whenever you quote or discuss previously published work, you should use the present tense; you are quoting established knowledge. You would say this just as you would say "The Earth is round." (If previously published results have been proven false by later experiments, the use of past rather than present tense would be appropriate.)

Your own present work must be referred to in the past tense. Your work is not presumed to be established knowledge until after it has been published. If you determined that the optimal growth temperature for Streptomyces everycolor was 37°C, you should say "S. everycolor grew best at 37°C." If you are citing previous work, possibly your own, it is then correct to say "S. everycolor grows best at 37°C."

In the typical paper, you will normally go back and forth between the past and present tenses. Most of the Abstract should be in the past tense, because you are referring to your own present results. Likewise, the Materials and Methods and the Results sections should be in the past tense, as you describe what you did and what you found. On the other hand, much of the Introduction and much of the Discussion should be in the present tense, because these sections often emphasize previously established knowledge.

Suppose that your research concerned the effect of streptomycin on Streptomyces everycolor. The tense would vary somewhat as follows.

In the Abstract, you would write "The effect of streptomycin on S. everycolor grown in various media was tested. Growth of S. everycolor, measured in terms of optical density, was inhibited in all media tested. Inhibition was most pronounced at high pH levels."

In the Introduction, typical sentences might be "Streptomycin is an antibiotic produced by Streptomyces griseus (13). This antibiotic inhibits the growth of certain other strains of Streptomyces (7, 14, 17). The effect of streptomycin on S. everycolor is reported in this paper."

In the Materials and Methods section, you would write "The effect of streptomycin was tested against S. everycolor grown on Trypticase soy agar (BBL) and several other media (Table 1). Various growth temperatures and pH levels were employed. Growth was measured in terms of optical density (Klett units)."

In the Results, you would write "Growth of S. everycolor was inhibited by streptomycin at all concentrations tested (Table 2) and at all pH levels (Table 3). Maximum inhibition occurred at pH 8.2; inhibition was slight below pH 7."

In the Discussion, you might write "S. everycolor was most susceptible to streptomycin at pH 8.2, whereas S. nocolor is most susceptible at pH 7.6 (13). Various other Streptomyces species are most susceptible to streptomycin at even lower pH levels (6, 9, 17)."

The principal exception to this rule is in the area of attribution and presentation. It is correct to say "Smith (9) showed that streptomycin inhibits S. nocolor." It is also correct to say "Table 4 shows that streptomycin inhibited S. everycolor at all pH levels." Another exception is that the results of calculations and statistical analyses should be in the present tense, even though statements about the objects to which they refer are in the past tense; e.g., "These values are significantly greater than those of the females of the same age, indicating that the males grew more rapidly." Still another exception is a general statement or known truth. Simply put, you could say "Water was added and the towels became damp, which proves again that water is wet." More commonly, you will need to use this kind of tense variation: ''Significant amounts of type IV procollagen were isolated. These results indicate that type IV procollagen is a major constituent of the Schwann cell ECM."

Active Versus Passive Voice

Let us now talk about voice. In any type of writing, the active voice is usually more precise and less wordy than is the passive voice. (This is not always true; if it were, we would have an Eleventh Commandment: "The passive voice should never be used.") Why, then, do scientists insist on using the passive voice? Perhaps this bad habit is the result of the erroneous idea that it is somehow impolite to use first-person pronouns. As a result, the scientist typically uses such verbose (and imprecise) statements as "It was found that" in preference to the short, unambiguous "I found."

I herewith ask all young scientists to renounce the false modesty of previous generations of scientists. Do not be afraid to name the agent of the action in a sentence, even when it is "I" or "we." Once you get into the habit of saying "I found," you will also find that you have a tendency to write "S. aureus produced lactate" rather than "Lactate was produced by S. aureus." (Note that the "active" statement is in three words; the passive requires five.)

You can avoid the passive voice by saying "The authors found" instead of "it was found." Compared with the simple "we," however, "the authors'' is pretentious, verbose, and imprecise (which authors?).

I am a scientist trained in the old school, in which there is a very rigid ethic of writing only in the passive voice. The more lax modern interpretation of this is to write "we" attributing the whole work to a team and not claiming personal credit.

However, your thesis is meant to be "an original contribution to human knowledge" - it has to be your own and solely your own work (unless it is part of a funded group research project). Therefore it is acceptable, indeed necessary, to credit yourself by saying "I" did this on some occasions in the thesis, just don't overdo it.

In virtually every thesis I examine these days I have to add some minor correction involving "change 'we' to 'I' on pages . . . ."

But in your supporting publications, don't forget to follow the rule passive voice (prefereably) or "we" - and ignore Microsoft Word's dumbed down spelling checker that tells you to avoid the passive voice - it's just impoverishing the language.

My understanding is that the reason for the passive voice construction is to take both author and audience out of the manuscript, so the objective story dominates. 

I recommend George Orwell's rules for writing:

* Never use a metaphor, simile, or other figure of speech which you are used to seeing in print.

* Never use a long word where a short one will do.

* If it is possible to cut a word out, always cut it out.

* Never use the passive where you can use the active.

* Never use a jargon word if you can think of an everyday English equivalent.

* Break any of these rules sooner than say anything outright barbarous. 

"Many books and guides on scientific writing currently advice authors to use the active voice whenever possible. The main argument is that the active voice helps to make the writing clear and concise. This represents a change from the situation in the major part of the 20th century, when the passive voice was characteristic of scientific writing in general. "

(Leong Ping Alvin, The passive voice in scientific writing. The current norm in science journals, Journal of Science Communication, SISSA – International School for Advanced Studies, 2014)

Thanks everyone for your answers.They are very helpful.

Based on your experience , do you see active in present tense more suitable than past tense. for example

we argued that model xyz is better than y2.  or we argue that xyz is better than y2.

experiments using method1 gave poor result, or experiments using method1 give poor result.

Issa

Below are extracts from:

Robert A. Day, How to write and publish a scientific paper, Oryx Press, 5th edition, 1998.

Chapter 32

Use and Misuse of English

Tense in Scientific Writing

There is one special convention of writing scientific papers that is very tricky. It has to do with tense, and it is important because its proper usage derives from scientific ethics.

When a scientific paper has been validly published in a primary journal, it thereby becomes knowledge. Therefore, whenever you quote previously published work, ethics requires you to treat that work with respect. You do this by using the present tense. It is correct to say "Streptomycin inhibits the growth of M. tuberculosis (13)." Whenever you quote or discuss previously published work, you should use the present tense; you are quoting established knowledge. You would say this just as you would say "The Earth is round." (If previously published results have been proven false by later experiments, the use of past rather than present tense would be appropriate.)

Your own present work must be referred to in the past tense. Your work is not presumed to be established knowledge until after it has been published. If you determined that the optimal growth temperature for Streptomyces everycolor was 37°C, you should say "S. everycolor grew best at 37°C." If you are citing previous work, possibly your own, it is then correct to say "S. everycolor grows best at 37°C."

In the typical paper, you will normally go back and forth between the past and present tenses. Most of the Abstract should be in the past tense, because you are referring to your own present results. Likewise, the Materials and Methods and the Results sections should be in the past tense, as you describe what you did and what you found. On the other hand, much of the Introduction and much of the Discussion should be in the present tense, because these sections often emphasize previously established knowledge.

Suppose that your research concerned the effect of streptomycin on Streptomyces everycolor. The tense would vary somewhat as follows.

In the Abstract, you would write "The effect of streptomycin on S. everycolor grown in various media was tested. Growth of S. everycolor, measured in terms of optical density, was inhibited in all media tested. Inhibition was most pronounced at high pH levels."

In the Introduction, typical sentences might be "Streptomycin is an antibiotic produced by Streptomyces griseus (13). This antibiotic inhibits the growth of certain other strains of Streptomyces (7, 14, 17). The effect of streptomycin on S. everycolor is reported in this paper."

In the Materials and Methods section, you would write "The effect of streptomycin was tested against S. everycolor grown on Trypticase soy agar (BBL) and several other media (Table 1). Various growth temperatures and pH levels were employed. Growth was measured in terms of optical density (Klett units)."

In the Results, you would write "Growth of S. everycolor was inhibited by streptomycin at all concentrations tested (Table 2) and at all pH levels (Table 3). Maximum inhibition occurred at pH 8.2; inhibition was slight below pH 7."

In the Discussion, you might write "S. everycolor was most susceptible to streptomycin at pH 8.2, whereas S. nocolor is most susceptible at pH 7.6 (13). Various other Streptomyces species are most susceptible to streptomycin at even lower pH levels (6, 9, 17)."

The principal exception to this rule is in the area of attribution and presentation. It is correct to say "Smith (9) showed that streptomycin inhibits S. nocolor." It is also correct to say "Table 4 shows that streptomycin inhibited S. everycolor at all pH levels." Another exception is that the results of calculations and statistical analyses should be in the present tense, even though statements about the objects to which they refer are in the past tense; e.g., "These values are significantly greater than those of the females of the same age, indicating that the males grew more rapidly." Still another exception is a general statement or known truth. Simply put, you could say "Water was added and the towels became damp, which proves again that water is wet." More commonly, you will need to use this kind of tense variation: ''Significant amounts of type IV procollagen were isolated. These results indicate that type IV procollagen is a major constituent of the Schwann cell ECM."

Active Versus Passive Voice

Let us now talk about voice. In any type of writing, the active voice is usually more precise and less wordy than is the passive voice. (This is not always true; if it were, we would have an Eleventh Commandment: "The passive voice should never be used.") Why, then, do scientists insist on using the passive voice? Perhaps this bad habit is the result of the erroneous idea that it is somehow impolite to use first-person pronouns. As a result, the scientist typically uses such verbose (and imprecise) statements as "It was found that" in preference to the short, unambiguous "I found."

I herewith ask all young scientists to renounce the false modesty of previous generations of scientists. Do not be afraid to name the agent of the action in a sentence, even when it is "I" or "we." Once you get into the habit of saying "I found," you will also find that you have a tendency to write "S. aureus produced lactate" rather than "Lactate was produced by S. aureus." (Note that the "active" statement is in three words; the passive requires five.)

You can avoid the passive voice by saying "The authors found" instead of "it was found." Compared with the simple "we," however, "the authors'' is pretentious, verbose, and imprecise (which authors?).

Tenses are another kettle of fish. Use the appropriate tense for the point on the timeline you are writing about and use it consistently. I spend large amounts of time trying to help my students use tenses correctly in their theses, but it's better than having an external throw it back and say "Correction is one thing, but it's not my job to proof-read this for you." The thesis is the most important thing you will ever write and it's worth doing it properly (unless you get a Nobel prize later).

Exclusive use of the passive voice in scientific writing is an antiquated and simplistic notion.  In general, most writers will agree that the active voice provides greater clarity and directness to a composition compared to the passive voice.  However, I suggest that you do not place constraints on your writing by limiting the use of one voice over the other; instead, write with optimal clarity and use all of the grammatical tools and words of the lexicon at your your disposal to keep your writing fresh and interesting.