Some agencies will be reluctant to share the information you seek, especially for higher-level jobs. If the person you contact is unwilling to participate, don't force the issue. Suggest that another time might be better, and move on. If a source requests confidentiality as a condition for cooperating, you might offer to present the survey results in code. Each participating organization should know who is included in the survey, but actual data would be shown as that of Agency A, Agency B, Agency C, etc.
Codes are easy to decipher, however, unless there are many participants in the survey.
Use of codes also demands that the survey organizer be perceived as trustworthy enough not to reveal the code to unauthorized persons. In most cases, a request for confidentiality is not motivated by a paranoid desire for secrecy but, rather, by a need for reassurance that data will be used by responsible people. Generating that confidence is another important reason to make personal contact before a salary survey begins. Before contacting potential survey participants, compile a list of positions you plan to include in the survey so that you can confirm mutual interest in those jobs. Others may be added as a result of your conversations.