The general rule for Polish surnames is that male surnames end in i and female surnames end is a. Thus, Joseph Wojciechowski would be the correct form for a man and his sister would be Mary Wojciechowska. I have found that in the USA this distinction is not always observed and you should search under both spellings. The census birth dates are also frequently inaccurate so you may have the right person. I hope this helps. I have several Wojciechowski/a ancestors in the Chicago area.