unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TfrmHillSlopeRegr = class(TForm)
    Save: TButton;
    Estimate: TButton;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure SaveClick(Sender: TObject);
    procedure EstimateClick(Sender: TObject);
  private
    // declaring your variable here makes the variable visible for this
    // TfrmHillSlopeRegr object instance
    OutputFile: TStringList;
  public
    { Public declarations }
  end;

var
  frmHillSlopeRegr: TfrmHillSlopeRegr;

implementation

{$R *.dfm}

procedure TfrmHillSlopeRegr.FormCreate(Sender: TObject);
begin
  // create the string list instance at form creation (in the OnCreate event
  // method of your form)
  OutputFile := TStringList.Create;
end;

procedure TfrmHillSlopeRegr.FormDestroy(Sender: TObject);
begin
  // free the string list at form release (in the OnDestroy event method of
  // your form)
  OutputFile.Free;
end;

procedure TfrmHillSlopeRegr.SaveClick(Sender: TObject);
begin
  // here, the OutputFile variable is accessible
  // OutputFile.Save(...);
end;

procedure TfrmHillSlopeRegr.EstimateClick(Sender: TObject);
begin
  // and here, the OutputFile variable is accessible as well
  // OutputFile.Add(...)
end;

end.