Link of the source where I found the phrase "in certain degree", but the only what I found on a dictionary is "to certain degree".
What is the difference between "to certain degree" and "in certain degree"?
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I am a native English speaker, and I have never heard the phrase "in a degree", unless you are talking about going to school to earn one. Example: "I am interested in a degree program that will help me advance my career." Google also seems to be backing me up here.
Similarly for "in certain degree", the only usage I have seen is something akin to, "In certain degree programs, level 4 math is a required prerequisite."
While it is possible that someone will point out some additional marginal use of the phrase, in nearly every other context than the example I gave, "in certain degree" or "in a degree" is simply incorrect.
Now, "a certain degree" is a different matter, and you can have something result in a certain degree of.... This is basically synonymous with "some".
Edit: I have, upon further searching, found some older English that uses the phrase "in certain degree" as part of academic writing. It is no longer part of the modern language, but the phrase does exist: http://www.linguee.pe/ingles-espanol/traduccion/in+a+certain+degree.html
This may be incorrect, but as far as i've experienced it, "to a certain degree" is used with regard to extent; "in a certain degree" is used with regard to manner. In other words, using "to" answers the question "how much?" and in answers the question "how?"
"The treatment was successful, but only to a certain degree."
In other words, the treatment was mostly successful, but there was some variable margin of failure; it wasn't completely successful.
"The treatment was successful, in a degree."
The operation was successful by one person's standards, but perhaps not another person's standards— it wasn't universally successful, but seemed to be satisfactory to at least one person.