Omitting word(s) when typing
In his 2003 psycholinguistics book, John Field has summarized (pp. 70-72) his own typing errors and combined them with an older corpus of Hotopf.
Missing words were among the frequent errors, but alas no numerical frequencies are given.
But what he says he noticed is that short function words like &quot;are&quot; or &quot;it&quot; (these are his examples) are more likely to be omitted. And regarding the type of words more likely to be omitted he says:

An important factor appears to be the difference between lexical and function words.
The latter seem very susceptible to error: they are sometimes omitted or duplicated, and it is quite common in rapid typing for one function word to be
substituted for another, even when they are not phonologically identical. This
leads one to the conclusion that function words may be processed differently from
lexical ones. It seems possible that the writing process accords a higher level of attention to lexical items.

Although he doesn't say this, the full-context examples that he gives for omission seem to have another property, namely that short words get omitted close to each other (compare with duplication)


though these [are] not frequent

that [it] is possible



And if you're curious about the related issue(s) he mentions, among the frequent duplication examples are &quot;the the&quot;, &quot;that that&quot;, and &quot;of of&quot;. Among the substitutions are &quot;the&quot; instead of &quot;that&quot;, &quot;of&quot; &lt;- &quot;for&quot;, &quot;doe&quot; &lt;- &quot;do&quot;, &quot;were&quot; &lt;- &quot;where&quot;, &quot;for&quot; &lt;- &quot;from&quot;. He continues:

Finally, notice that some writing slips provide evidence of a planning process operating
at a level higher than the word. When writing a particular word, we already
have the next few words stored in our minds, ready for production

The examples for these are more interesting


importan topic
these given (= give) insights
using bothing (= both) approaches
difference (= different) intelligence tests
neighbourshoods


This is more qualitative than quantitative, but for lack of anything better, I put it in the answer box.
So if Field is correct, there's an anticipatory process in writing that causes a number of spelling errors, not all of them omissions.