word choice - Thesis writing: how to address "self"

This came up when I was writing my thesis:

Usually it is recommended that when writing a thesis, one sticks to passive voice and use sentences that read like "The study showed the effect that blah blah has on the blah of blah" and so on.

In certain situations, one needs to mention something analogous to "Through this study we showed blah blah". In such a situation, how does one go about writing it? The thesis is supposed to be an individual's attempt and so the usage of "we" seems inappropriate. In the same way, use of "I" is frowned upon. How does one write sentences like these?

Using alternates like

"Through this study the author showed"

"This study showed"

seem a little redundant because of over usage of "the author" or "the study". Are there other alternatives?

Further, in a paper one can write sentences like "We believe that the effect of X on Y is because of the presence of Z" but how does one write similar sentences in a thesis?

Answer

I often run into this problem too. I think in the end it usually sounds redundant anyways but I use phrases like "the data suggest" or "the results suggest" in the discussion and in the introduction I usually just state the claim without attributing it to myself since it's assumed it is "this study" (unless it's cited information). You don't technically have to attribute the statements unless there things you citing from your previous work or others works. Other times it's useful to just write the sentence and then edit it like this:

"Through this study we showed the influence of three iron oxides concentration on soil color" becomes "The concentration of these three iron oxides influences soil color by blah" or "The influence of these three iron oxides concentration on soil color is blah." Which makes the sentence more concise and direct while removing any need of attribution.

When referring to other authors work I generally just rely on the in-text citation but sometimes I'll say something like "has been described by (Authors)" or "hypotheses developed by (Authors) suggest" or "a (year) study by (Authors) concluded that."

As for the last sentence I would probably write something like "The effect of X on Y appears to be influenced by the presence of Z which is supported by the data (chart or table or statistical output)" or "Z influences interactions between X and Y by blah."

As a side note, avoid making statements that affirm definite causation like "is because of" or "prove" and instead use some form of suggests or supports/is supported by. And this may be just a stylistic choice but I try not to use show/shows/showed.