hello , I have two data set and just one variable which is the code of the country is the same. when i use m:1 or even 1:m I receive that variable "pifscode does not uniquely identify observations in the master data" or "even variable pifscode does not uniquely identify observations in the using data" , would you please help me:
Code:
year    GDP    ln_GDP    wpui_CODE    wpui_pa    WUI_CODE    wui_pan    WTUI_CODE    wtu_pa    merge2    merge1    merge3    pifscode
1990    .27234152    -1.3006984            111    .395295            master only (1)    using only (2)    matched (3)    111
1973    .25884304    -1.3515334            111    .0897666            master only (1)    using only (2)    matched (3)    111
2020    .23573394    -1.4450515    111    66.45792    111    1.4796039    111    41.016245    matched (3)    matched (3)    matched (3)    111
1980    .28120953    -1.2686552            111    .248631            master only (1)    using only (2)    matched (3)    111
1983    .28044554    -1.2713757            111    .3962155            master only (1)    using only (2)    matched (3)    111
1971    .25592745    -1.3628613            111    .4107295            master only (1)    using only (2)    matched (3)    111
2019    .23573394    -1.4450515    111    0    111    2.1541754    111    145.61622    matched (3)    matched (3)    matched (3)    111
2006    .25921943    -1.3500804    111    0    111    .2388608    111    0    matched (3)    matched (3)    matched (3)    111
Code:
logTradeij    LogYi    LogYj    LogDistanceij    date    ifscode    pifscode    countryi    partnercountryj    countryi_    partnercountryj_
-10.966815    31.093663    29.193731    34.500716    1996    111    112    United States    United Kingdom    United States    United Kingdom
-10.775703    31.26808    29.345101    34.500716    1997    111    112    United States    United Kingdom    United States    United Kingdom
-10.651133    31.438519    29.483399    34.500716    1998    111    112    United States    United Kingdom    United States    United Kingdom
-10.750845    31.62707    29.607866    34.500716    1999    111    112    United States    United Kingdom    United States    United Kingdom
-10.859496    31.789523    29.774053    34.500716    2000    111    112    United States    United Kingdom    United States    United Kingdom
-10.71019    31.832637    29.887876    34.500716    2001    111    112    United States    United Kingdom    United States    United Kingdom
-11.032036    31.904512    29.983911    34.500716    2002    111    112    United States    United Kingdom    United States    United Kingdom
-11.168279    32.004573    30.133492    34.500716    2003    111    112    United States    United Kingdom    United States    United Kingdom
-11.442545    32.141164    30.2485    34.500716    2004    111    112    United States    United Kingdom    United States    United Kingdom

I hope I could receive your guidance so soon. thank you so much.