1. A letter/pack of pre-notification to be sent by regular mail to the postal address of the investigator. This should contain
   • a Cochrane leaflet and, at the discretion of the individual reviewer, details of the Cochrane Review Group to which he or she belongs;
   • the protocol, or a brief version of the protocol, for the current review;
   • the citation(s) so far obtained for the investigator's study with a comment concerning its qualities or importance to the systematic review;
   • a request that extra or new contact details (telephone/email address), or any new contact person for the trial, be forwarded to the reviewer as soon as possible;
   • advance notification of a forthcoming request for further information;
   • an invitation to contact the reviewer with any questions regarding the review. Multiple modes of reply should be offered to the investigators, including address, telephone, email and fax.
   An example covering letter appears here.

   This stage of the intervention has three specific objectives: (i) to allow the investigator to initiate the retrieval of information for their study; (ii) to identify, if necessary, more appropriate contact information for the study; and (iii) to improve the likelihood of response to the main letter by providing pre-notification and information.

2. If the investigator responds to the pre-notification, the reviewer may enter into correspondence of any type with the investigator in order to retrieve the missing information by the most convenient means. If no reply is forthcoming then the reviewer should proceed to number 3.
3. A letter requesting the missing information, posted three weeks after the pre-notification, containing
   • a reminder of the pre-notification;
   • the citation(s) so far obtained for the investigator's study;
   • a form or table (perhaps the data extraction form being used for the review), which has been completed as far as possible by the reviewer. This should make clear which pieces of information are missing or unclear from the published report. It may be helpful to enclose an uncompleted data collection form for the investigator to fill in;
   • notification that all contributions will be acknowledged;
   • notification that investigators approached by the reviewer will be mentioned in the review irrespective of any response;
   • an invitation to contact the reviewer with any questions regarding the review.
   Multiple modes of reply should again be offered to the investigators, including address, telephone, email and fax. An example covering letter appears here.

   This stage of the intervention aims to (i) confirm the accuracy of the extracted information; (ii) request missing information; and (iii) improve the likelihood of response by promising acknowledgement in the final report.

4. An attempt to follow-up investigators who have not replied approximately four to six weeks after the letter of request. This may be by any means available to the reviewer. We encourage the use of telephone and email. This stage of the intervention aims to further improve the response rate by reminding investigators who may have forgotten to reply, by increasing the speed of any response, and by increasing the likelihood that correct contact information will be obtained if the postal address for the investigator was incorrect.

Email version
A multi-stage approach to requesting data or information consisting of

1. A letter of pre-notification to be sent by email to the investigator. It is inadvisable to send email attachments without the consent of the recipient. The body of the email should therefore contain
   • a brief introduction to the Cochrane Collaboration, with links to web sites for further details;
   • an offer to send further details of the protocol for the current review;
   • the citation(s) so far obtained for the investigator's study with a comment concerning its qualities or importance to the systematic review;
   • a request that extra or new contact details (telephone/surface mail address), or any new contact person for the trial, be forwarded to the reviewer as soon as possible;
   • advance notification of a forthcoming request for further information;
   • an invitation to contact the reviewer with any questions regarding the review. Multiple modes of reply should be offered to the investigators, including surface mail, telephone, email and fax. The example covering letter in Appendix C may be used as a template. This stage of the intervention has three specific objectives: (i) to allow the investigator to initiate the retrieval of information for their study; (ii) to identify, if necessary, more appropriate contact information for the study; and (iii) to improve the likelihood of response to the main letter by providing pre-notification and information.
2. If the investigator responds, the reviewer may enter into email, or other, correspondence with the investigator in order to retrieve the missing information by the most convenient means. If no reply is forthcoming then the reviewer should proceed to number 3. If the email attempt is unsuccessful (that is, 'bounces' because the email address no longer exists) then the reviewer is encouraged to immediately initiate the experimental arm using surface mail. The surface mail option may produce a beneficial outcome since the letter may still reach the investigator (since email addresses may change more frequently than postal addresses), or the letter may be forwarded or opened by a colleague. If the reviewer prefers, attempts to contact the investigator may cease, in which case the outcome for this investigator will be 'failure to contact'.
3. A letter to be sent by email requesting the missing information, posted three weeks after the pre-notification, containing
   • a reminder of the pre-notification;
   • the citation(s) so far obtained for the investigator's study;
   • the request for missing data;
   • notification that all contributions will be acknowledged;
   • notification that investigators approached by the reviewer will be mentioned in the review irrespective of any response;
   • an invitation to contact the reviewer with any questions regarding the review. Multiple modes of reply should again be offered to the investigators, including surface mail, telephone, email and fax. An example covering letter appears in Appendix D. This stage of the intervention aims to (i) confirm the accuracy of the extracted information; (ii) request missing information; and (iii) improve the likelihood of response by promising acknowledgement in the final report.
4. An attempt to follow-up investigators who have not replied approximately four to six weeks after the letter of request. This may be by any means available to the reviewer. We encourage the use of telephone. This stage of the intervention aims to further improve the response rate by reminding investigators who may have forgotten to reply, by increasing the speed of any response, and by increasing the likelihood that correct contact information will be obtained if the email address for the investigator was incorrect.