这个availability是指能不能参加，或者说何时可以参加，何时不能参加等情况。我觉得这里不宜直译，应作变通。可以翻译成“如果您参加研究的计划发生了变化。。。”或“如果影响您参加研究的客观情况发生了变化。。。”

“您是否有空参加这个研究”我觉得是这个意思吧，直接翻译出来会比较生硬的。
谍影重重3里面演的，Pamela Landy通过接线员CIA的boss打电话，boss正在开会，给接线员说的：tell her that I am not available。

The English word is a noun. But in Chinese, it is very difficult to to make the expression into a noun. And the best way is just forget that "if possible a noun should be used."
Sticking close to the English and sacrificing fluency in Chinese: 对于参加这项研究工作，你能否参加，愿不愿意参加的情况有没有改变？
In better and more fluent Chinese:　　你是否仍旧能够并且愿意参加这项研究工作？
Available -- two essential elements: (1) the participant's willingness and (2) if it is possible for him/her to participate (such as has the time, not otherwise occupied, physically capable, etc.) Only the participant makes the decision on availability. Translate it into "suitable, appropriate" and/or "convenient" seems to be a bit off. They could be interpreted as
"suitable, convenient" to all parties, a value judgment.


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Note added at 2 hrs (2012-05-31 06:24:42 GMT)
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Of the two elements (willingness, possibility/capability) people usually give greater weight to the latter (possible, having the time, not otherwise occupied). But willingness is equally important. If you do not wish/want to answer the phone, you would also tell the operator: "Tell the caller:t I am not available." If you really cannot answer a call from your young child, you'll probably say: "tell her that I will call her 15 minutes later.,,"or something like that.

如果直译就是如果您参加本研究的availability发生变化，请通知医生，非常拗口，事实上根本没法翻
可以变通一下，从逻辑上，availability变化无非就是available变为unavailable和unavailable变为available，后一种情况不符合实际，因为如果患者已经确认unavailable，医生是不会与其联系的，所以选择第一种情况
因此可以翻译成，如因故不能参加研究，请通知医生

有时间参加

your availability的變化無非就是不能在原定的時間參加，
available 能按時參加

become unavailable，因為出現變故而變得不能按時參加

這裡沒有中間地帶， 只有YES OR NO。

如果你在原定的時間無法見醫生，就應該通知他，他會決定是取消預定的那次用藥(或者抽血，體檢，回答問卷等)