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In this sentence:

> わたしは、ほぼほぼ計画通りに過ごすことができた一年でした。

Does the に particle make "計画通り" an adverb?
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particle-に adverbs
Is に optional in this sentence?
> Is に optional in this sentence?

No. It's necessary.

> Does the に particle make "計画通り" an adverb?

Yes. The 「に」 particle has a lot of usages and makes a noun (or a noun-like
phrase) adverbially connect to the verb in different ways.

In this case, No. 8 of [this dictionary
entry](http://dictionary.goo.ne.jp/jn/166083/meaning/m1u/%E3%81%AB/) the most
matches:

> 動作・作用の行われ方、その状態のあり方を表す。「直角に交わる」「会わずに帰る」  
>  _Indicates the manner in which an act is done, an effect is made or a state
> is being._

So 「計画通りに」 means something like "in a manner that is 計画通り (just as planned)."
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answer
Yes it's optional. The following sentences are both correct and feel exactly
the same to me.

>   * ほぼほぼ計画通りに過ごすことができた一年でした。
>   * ほぼほぼ計画通り過ごすことができた一年でした。
>

に is not necessary in [certain standalone
adverbs](https://japanese.stackexchange.com/a/41536/5010), and it's optional
in some adverbial phrases, too. ～通り is a phrase where に is optional. E.g.,
予定通り(に)通過する, 予告通り(に)放送する, 練習した通り(に)やる.

You should remember when に is optional to answer a question like
[this](https://japanese.stackexchange.com/q/42328/5010).

See also:

  * [Difference between くらい and くらいに](https://japanese.stackexchange.com/q/42280/5010)
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answer